Damn Small Linux can run a graphical desktop environment with as little as 16 MB of RAM (although 24 is recommended).

That really makes me want to see a Wii with a mouse and keyboard plugged in displaying a spreadsheet or something. Unfortunately DSL only supports x86. Theoretically it could be ported to PPC like Void Linux was, though I don't know if all the tweaks they did to make the kernel and pre-installed packages as small as possible would make that harder.