The N900 seems to be fulfilling every geek’s dream! With each passing day we see something awesome come out, first it was Windows 3.1, then we saw Windows NT 4.0, next was Windows 95, followed by Mer, Debian, Android and now even Windows 98. That is seven different operating systems apart from its very own Maemo 5.
While it is true that most of the OS’s above are more ‘proof of concept’ rather than being actually usable, it still goes to show the awesome power the N900 packs. The one exception to this is ‘Mer’ that is being developed as an alternative OS for the N900, you can find much more info about it here. You can install any of them without having to loose out on Maemo 5 as they either run in an emulator or can be used in a dual-boot mode.
Android: No install instructions are available, yet.
Windows NT 4.0: Windows NT 4.0 running under Bochs 2.3.7 PC emulator on a Nokia N900 phone. More info here.
Windows 3.11: Run in Dosbox.
Windows 95: Done via Dosbox.
Debian: Complete video tutorial below.
Mer: Is a mobile Linux distribution, complete instructions on how to get it on your N900 are here.