There is a community of Mac users that try to find the balance between having the best tool for the job, keeping their machines useful for as long as possible to get the lowest possible cost of ownership over time and staying efficient by not disrupting well working workflows. Most of these are aware that unless you work with high end video editing, play the newest and most demanding games or work with extremely demanding number crunching, you do not really need the most powerful computer out there, but can get by happily with lower end machines. Many buy last year’s model refurbished from Apple, buy used Macs or buy the newest machine, but keep using it for years.
However, there is a problem in Low End Mac land and that is security. When your machine is more than two versions behind the newest macOS, Apple no longer give you security updates for your machines. In the past, the “if it ain’t broke” approach worked well since Macs were not browsing the internet, but these days, not getting security updates can be catastrophic. An example is that all machines that runs anything older than Mavericks have the infamous “heartbleed” vulnerability in their versions of bash. These machines are exploitable when used on public wifi or if their home router’s firewall is not set up correctly (or is itself vulnerable, as many non-upgradable home routers are). It helps to turn on the firewall inside Mac OS X and to use an up to date browser (which is seldom available for older version of Mac OS X), but there are no guarantees that you will stay secure.
The real problem from the low end point of view is Apple’s business model. Apple primarily earn money by selling hardware. If there are any technical reasons not to let older machines get newer versions of the OS, they do not hesitate to block older machines from upgrading. This means that some machines get quite a short period of Mac OS X updates, while others get a much longer period of support. Generally, lower end machines are cut off earlier, as they often do not have the CPU, GPU or RAM that their mid range or high end siblings have. For example a white MacBook from late 2006 is still a decent machine to surf the web with, play some older games on, write some emails, do word processing and most of the things most computer users do, but it has not gotten the ability to upgrade to newer versions of Mac OS X than Lion and since even Lion is more than two versions behind by now, users of these machines cannot get security updates if they continue to use Mac OS.
This is where Linux comes into the picture. Older Macs, both with PowerPC and Intel processors, can run Linux quite well and get the newest software years after Apple stopped supporting them and browser makers stopped supporting the newest versions of Mac OS X they can run. Take the late 2006 MacBook again. With Mac OS X Snow Leopard, this machine no longer gets security updates and there is no currently up to date browser with the newest security fixes (which is important for browsers since they are internet-facing), but with Ubuntu on the same machine, you can run the newest version of Firefox, Chrome, Chromium, Vivaldi, Opera, Qupzilla… or whatever browser you like and the operating system and every other program is getting security updates. With Linux, your old “obsolete” Mac is no longer obsolete. Instead, it is useful, secure and up to date.
There is a community of Linux users with the same ideals as the Low End Mac users. Most of these use older PCs, but some use older Macs. Even really old PCs and Macs that cannot run any supported version of Windows or Mac OS X can often run lightweight Linux distributions. (Debian 8 with a lightweight desktop environment runs, well, walks, on my iBook Clamshell with its 300 MHz G3 processor and 768 MB RAM.) In addition to the lightweight distros like Lubuntu, Ubuntu MATE, LXLE, Peppermint OS, Bohdi and Debian with LXDE or Openbox, there are also some ultra light weight distros specifically targeted at older or low end machines, such as AntiX, Puppy Linux and Damn Small Linux that will give you as much speed as possible out of your old machine.
On the Mac side, there are a number of blogs and web pages where people discuss using Linux and/or Mac OS X on older Macs. PPCLuddite is one of the most up to date Linux-focused Power Mac blogs. Linux on your Apple Mac also offers some information about this, and the developer blog for TenFourFox, even if focused on porting FireFox to Mac OS X Tiger on PowerPC, also occasionally gives insight into FireFox on Linux on PowerPC. For info on running Ubuntu on PowerPC Macs, have a look at the PowerPC FAQ and the PowerPC known issues. Ubuntu MATE and Lubuntu are the two official flavours of Ubuntu that still make PowerPC installation media available. Debian has also been a winner on PowerPC hardware with an official install guide here. However, the next version (Debian 9) will not support PowerPC Macs. (MintPPC used to be an alternative as well, but the last version, 11, is old and no longer gets security updates, so stay away from it.)
Since Intel Macs are AMD64 PCs with a few minor differences, many distros work on these. Especially Ubuntu based distros have an installer that is usually successful on Intel Macs in my experience, even the older ones with 32 bit EFI and 64 bit processors. You just have to use rEFInd or rEFIt before installing, but you can uninstall these after installing your Linux distro if you like. Ubuntu has the Mactel support page with more information and there is information on using Arch on Macs on the Arch Wiki. Just search for the Mac you have. You could also search the Debian Wiki for your Mac model if you want to use Debian on your Mac. I have had trouble installing Debian on my MacBooks (late 2006 and mid 2007) and my MacBook Pro (mid 2010) in the past, but this might have changed since I last tried a few years ago. (On PowerPC, Debian has worked well for me.) I have made a write-up of running Ubuntu 14.04 on a MacBook 2,1 on this blog, so if you have one of these machines, have a look at that blog post.
When does it make sense to use Linux on a Mac?
It does not make a lot of sense to buy a Mac to run Linux, so if you want new hardware to run Linux on, buy a machine with Linux preinstalled if you want the best experience, or buy a (used) Windows PC after researching a bit online to find out if it runs Linux well or not. On the other hand, if you already have a PowerPC or Intel Mac, then installing Linux alongside Mac OS (X) might make the machine more useful if it no longer gets support by Apple or if you want to try out Linux on real hardware. (The alternative is trying Linux in VirtualBox.)
PowerPC is no longer a priority for most distros and both Debian, Lubuntu and Ubuntu MATE will drop support for PowerPC in future releases, so if you want to try out Linux on these machines, Lubuntu 16.04, Ubuntu MATE 16.04 or Debian Jessie are your alternatives. Lubuntu 16.04 and Ubuntu MATE 16.04 will receive security support until 2019, so there is still a window of time where using Linux on PowerPC is possible. Beware that PowerPC Macs are not very powerful by today’s standards and will not give you the fastest Linux experience with their slow had drives. There are also some proprietary (ie non-open source) programs that do not work on PowerPC Linux even if it is otherwise available on Linux, such as Flash, Chrome and Skype. The resale value of PowerPC Macs are really low now, so it is little to gain from selling them, but on the other hand, slightly newer x86 PCs cost very little as well and will give you a better Linux experience for very little money, so maybe a sale of an old PowerPC Mac might give you enough to buy an old ThinkPad which might give you a much better Linux test machine. Just remember to research the Linux compatibility of used machines online before buying.
If you have an older Intel Mac and want to give it a new life, then Linux is a good option to squeeze out a few more months or years of useful life of an otherwise obsolete Mac by Apple’s standards, but bear in mind that your Linux experience could be better on hardware with better hardware support. Especially the older MacBooks have fans and webcams that need a bit of tweaking to work, and even then, the experience is not the greatest. (Install macfanctl and extract the iSight firmware by using the instructions in the Ubuntu Mactel documentation for the best experience.) That being said, it works well enough that I used a late 2006 MacBook with Linux as my laptop for a few years while also having the option to boot into Snow Leopard to use the few remaining Mac programs I did not have good Linux substitutes for at the time. The main drawback was a shorter battery life on Linux than on Mac OS X, but the main advantage was up to date software with a pleasant user experience and a new lease of life to the old workhorse.