Claws Mail as a mail client could be good...
Games... I just play open-source... If you like games like Worms, Wormux is good.
For IM, why don't you use the one Maemo has? It's really better than Pidgin!

PenPen, and Xournal for note-taking... EasyDebian, to have Debian in your pocket... Or if you don't know how to use it, to just open and use a few apps inside, and show it to others and show off! lol
Also, power kernel if you are a power user, and want to overclock maybe?

I suggest you overclock! There are many guides available... Here is one:
http://tinyurl.com/39zsldt

Also, I suggest VIM, QCPUFreq, etc.
If you actually know how they work and what they are for....

For painting, a great app, is mypaint!

Also, if you install EasyDebian, you'll have GIMP which is great!

I seriously suggest you install EasyDebian.... You can run Debian apps inside it! If you had questions regarding DEBIAN, you may ask them on Debian Forums or http://ask.debian.net/... But as I said, only Debian questions... Not ones related to N900...

Don't forget that your N900 is not a phone... It's a mobile computer... Don't forget to do whatever you can with it! lol
I mean, don't do what you can do with a 1100 with an N900... Use all features!

Good luck!

EDIT: Forgot some of my favs here:
ThemeCustomizer, Flashlight, LED pattern editor, cpu and memory usage applet, Carbon Theme, FM Boost, Airtux Sounds, bash 3, Bootscreen, Conky, CuteExplorer, Droid Fonts, Easy Chroot, eSpeak, Extra Decoders Support, fMMS, Here and Now, KMPlayer, Sib, ukeyboard, Wi-Fi Signal Applet, X-motto, Yellow Notes Applet, tweakr, Shariks, QtIrreco, qlister, Pidgin protocols plugin for Conversations and Contact, Personal Photo Frame, Nameday applet, Midnight commander, mbarcode, Mappero