In no particular order;

Texting - I love threaded conversations as when someone replies with something like 'yes' I often forget the question and have to go back and look at what I sent. Not anymore!

Games - I save the flash from http://wiiarcade.com (good website as no keyboard controls are needed for any game), and play it offline.

Media player - Most people aren't fond of watching movies on such a small screen, but I came from the n95 and I loved to watch movies on that, so the size is a nice step up. I'm searching for some way to carry the n900 so I can use it as my media player while running.

Web browsing - It's replaced my netbook too, only use my main PC for gaming and iPlayer really now (hopefully the latter can be a task for the n900). It also solves a few petty arguments by googling the answer when out and about.

That's the main uses, I also use it as a phone, reading the odd .pdf and .doc files, the calendar comes in handy from time to time, and like every phone i've ever used, it's also my main alarm clock.