I've installed a pile of apps, but I've gotta admit so far I mostly just use the pre-installed applications, they are pretty good. The downloaded apps I've used most frequently are:

Vim -- Maemo comes with a minimal vi editor built-in, but I just can't live without my infinite undo, colored text, syntax highlighting, etc.

Raemote -- remote control widget for Apple computers; easy way to control playback from iTunes.

WifiEye -- a nice quick view of the strength of local wireless access points (useful for hinting where they may have contention with one another as well).

Other apps that I rarely use, but are very cool:

Mappero -- amazingly powerful (and yet fully open) GPS-powered mapping program; works with several different map repositories (including Google maps)

Firefox Mobile -- more full-featured than Maemo's Microb browser, though Microb does everything I need so far...

Battle for Wesnoth -- While the iPod version of this game has been under construction for some time, and they are only starting to talk about adapting it for Android devices, someone simply compiled the raw desktop version of the game for the n900. And it works beautifully!

Kanatest -- as an Anime addict, I've been trying to learn Japanese for years. Kanatest only drills you on hiragana and katakana (no kanji), but (sadly) I still need that level of help...