Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
One of the reasons why one would get Linux is to make use of old hardware. Windows Vista, with all the DRMs built in, is bloated beyond belief, especially with the shiny "Aero" effects. Pretty but useless, like Paris Hilton... Anyways, people who do this had better check the system requirements before they pick a distribution. Ubuntu is the obvious choice for most people, but when that person is stuck on a 400MHz Pentium II laptop with 128MB of RAM, it might not be such a good idea. So, here are some small distributions that get the job done on old systems, too. They might also be good for gamers who want to eke every possible bit of performance out of their machine.
-Ubuntu Lite and Fluxbuntu: Variants on Ubuntu that use IceWM and Fluxbox, respectively, as their desktops. Although these are lighter than their GNOME counterpart, you can still do better- they come with bloated services that slow the system down.