This is a very USEFUL thread. I honestly mean it. It is easy to see applications in the repository but lazy and time consuming to check the details of the applications. So, this helped alot when all of you guys added the functions of the applications. I would love to add my list of favo/used applications but all of them are already mentioned, already. However, it might help someone, anyways.

1.3G/2G switcher - applet which helps you to switch your network from the status bar. (Saves battery)
2.Abiword - free word reader and writing app.
3.camkeyd - pressing the camera button minimizes the active desktop and takes to the the dashboard (helps when you browse the net)
4. Facebrick - Status update for Facebook. (Other Facebook app drained my battery)
5.FM radio Player - FM transmitter (useful when you want to listen to your N900 playlist in your car)
6.gnumeric - Free spreadsheet viewer.
7. headphone daemon - pauses your music when the headphone is unplugged.
8. Lense Cover reminder - Vibrates if you forget to close the camera lens cover.
9. maenotify - similar to call notifier.
10. Mstardict - Dictionary
11. Sharing Puglin for Picasa - upload pictures to Piacasa
12. Wifi switcher - an applet to switch on/off your wifi.
13. Touch search - Good search on desktop.
14. pycallblocker - If you have a crazy ex girlfriend, then block their unwanted calls. lol
15. OMweather - weather widget.

These are the apps, i use and think very important/useful for me.