There are so many Android apps that it's hard to keep track of some neat ones we've never heard of, so post away!
I'm looking for something similar to Mobile Defense since I cannot wait for it to re-open from closed beta. I wasn't impressed with the alternatives I've seen thus far (i.e. Lookout, Where's My Droid) which either have shoddy GPS tracking or lame SMS/codeword-centric functionality. So if you have something I don't know about yet, holler at your boy.
Anyway, some of the apps I'm using right now that I really love from left to right:
GOTO Android App
A really awesome screen lock app that notifies you of new messages, e-mails (Gmail only at this time) and missed calls while also displaying the time, date and weather.
Not really all that useful (especially for those that have mistresses ) but if you prefer to have a picture of your contact pop up along with a coordinating text message, then you'll love this.
There's like dozens of Twitter apps for Android and everyone has their own favorites from Twidroid to Seesmic, but personally I prefer Touiteur. I love reading my tweets in color coded fashion.
Tunewiki (not in photo)
Your typical no-nonsense music player with scrolling lyrics interspersed on the screen. I use it to access online radio stations like Last.FM <3