I only use my N900 as backup phone by now, even with its nostalgic bonus the system is a bit to old and slow for a daily driver. With Leste this might change, although I hope for Leste on a more modern device.

Still I have some apps on my N900 which I deem useful:

Chromium Browser: When others don't work

mBarcode: Barcode reading, fast and reliable.

Leafpad: Texteditor, as simple as it gets and just as useful.

PenPen: Quickly draw something to show it.
Or hand it to kids/grilfriends for doodling.

TimerJinni: All your simple timekeeping needs

Speedcruch: When you have to do real calculations

Pierogi: To mess with many TVs

Evopedia: Wiki at your hands even without network. useful when travelling, less useful year by year as the wiki images are severely outdated by now.

Games
ScummVM: For Monkey Island and other Lucasart Gems
Puzzles: Many little logic puzzles.
Wagic: Magic the gathering game which works for a limited number of cards up to the year 2011.

VNC Viewer: Well... for VNC

GPSjinni: To give you hope when the GPS takes to long to get a fix

And many others which I do not use anymore.
Marble Maps (unreliable), AGTL(broken), Cleven(WEP is rare now), Evilalarm(Teenager me needed it), FBReader(N9's OLED defeats the n900 by miles), Meecast, Modrana, Mypaint, Navit, Netstory, nQaap, Orechiette(808 Pureviews recording Quality is so much better), qMetro, SleepAnalyzer, Transmission

Edit: Forgot your map comment.

As said, marble (sometimes) works with Openstreetmap.
otherwise you can get Nokia Maps from here, they should work on the N900 too. However the last update for them was 2015.