Byline: Syncs 2-ways with Google reader, great for off line reading of image heavy content

Last.fm, Pandora, and Slacker: music discovery streaming apps

NPR News: Great interface for podcast content from National Public Radio

Instapaper: Download web pages for off line reading later

WiFiFoFum: non default wi-fi finder that allows you connect to signals not revealed under the default (may not be an issue with the N900)

A Google Voice interface, Cydia store has GV Mobile that I use with my Gizmo5 and Truphone accounts to make wi-fi calls on my iPod Touch

VNC, RDP

Not sure how the N900 map app will work, but Atlas from the Cydia store allows map caching so that you don't need a live data connection for GPS, like Maemo Mapper