Plenty. N900 runs on linux  so if u have knowledge in linux u can tweak this tweak tat all it need is the command or installer. Its still in development they even OC their phone from 600mz to 1200mhz ( at your own risk)  Try drop by a nokia store near you and try it.

1) Amazing graphics quite close to iphone
2) Amazing browser with flash supported no need 3rd party and no annoying lag
3) Amazing 5MP camera
4) Tons of application available to download from
5) Integration of Msn, Google talk, YM , QQ, etc etc without 3rd party but only plugin
6) Support additional decoder like RMVB,FLAC by installing decoder without 3rd party software
6) The screen is nHD.
7) 3.5 inch screen and visible under sunlight when 5800XM do not.
8) High virtual memory
9) Email integration no hassle just put your email address n password it will retrieve your setting from nokia server
10) Can play heavy flash files without lag
11) JHave  Xterminal http://en.wikipedia.org/wiki/X_terminal
12) Sms are sorted in conversation type.
13) Design to multi task without lag.
14) Able to sign in all your online messenger 1 click.
15) qwerty keyboard
16) ARM Cortex A8 600 MHz, PowerVR SGX530 graphics  ( Its like a pc with external graphic card better or internal)

and many many more

Softwares can be found @
a) Ovi stores
b) Maemo.org
c) and 3rd party website

Watch this video


For more info join us here
http://forum.lowyat.net/topic/1371100

It took about 1 year plus for 5800 or other nokia phone to get custom firmware imagine if n900 have 1 year with something so powerful imagine how far it can go.