Well, if you already had an n900, odds are you'll already know a lot of the favorite apps out there.

For myself, the most-used app on my n900s (other than the browser) would have to be gPodder, the podcast app. I probably couldn't live without "The History of Rome" (and various other history podcasts), as well as my collection of weekend political commentary podcasts.

I'm also a big fan of the remote-control app "Pierogi", but then I'm biased towards that one.