I thought I'll add my two cents to this thread, now that I've had the n900 for a few weeks. Consider, basically, the distinction between "Consumption" (Media, Web Browsing, Games), "Utility" (GPS, Flashlight), and "Production / Memory" (Text, photo, images, etc.) -- I was mostly drawn to the device because of the last factor.

So here are my picks for a beginner wanting to try something new:

VIM -- Yes, a glorified text editor, but basically the reason why (along with the hardware keyboard) why I bought -- or justified buying -- this device. The most amazingly useful scripting / text editor / personal wiki, etc. program in the world. I store everything I write in one text file, from essays, to bits from the web, to small scripts, etc. What's nice is that there is no real distinction between scripting / coding and data or writing.
I used to use a whole bunch of other programs, like Zim wiki, Maepad, etc. before I realized that Vim does everything. The killer tip for me was that "+Y Copies into the clipboard and "+P pastes from the clipboard.

Opera 11 -- MicroB is fine, but Opera 11 is that much faster and easier to use, despite there being a few bugs. I can use MicroB 100% of the time, I can use Opera 95% of the time but it's a far better experience, and I have MicroB around for the other 5%.

Wagic -- Amazing clone of magic the gathering that should not be legal and most likely technically isn't. Like everything else, takes some work to have working right, ie, so that it uses the card images.

MyPaint -- I don't actually have a reason to use this, but it is so incredibly cool for producing images that look painted, inked, or sketched. I'm honestly dying for a reason to use it (like maybe write somebody a hand-written email or something)

DropN900 -- For dropbox integration so all your files are backed up. Never worry about losing your device or corrupting your data again. Especially convenient if you want to try the weird dangerous stuff.
I've found that I NEED a convenient or automatic way to back up everything, otherwise I simply won't do it.

Dungeon Crawl Stone Soup -- No one has a ported the newest version, so you have to use SSH to play the newest releases online. Kind of silly, because it's not really a multi-player game, but still worth it.