I love all the ones posted, so I'll post a few different ones
1. better keyboard - lets you customize and apply skins to the virtual keyboards. you can have a nice simple black keyboard, or an iphone rep, or even a sports team. It also has a leet keyboard option lol
2. task list - very nice and simple app that allows you to set up categories, and list events and tasks under them, and then rank them by importance. which is great, I have it setup for repairs I want to make to my car, under private I have little thing I need to do while at home, school I can rank term papers and assignments, and for work...
very helpful app
3. Android Agenda Widget - my favorite agenda app. syncs with gmail, and looks awesome. also has 3 different sizes and you can edit almost everything appearance wise
4. speed test - simple connection strength test that gives you upload and download speeds similar to the one found online, great for determining whetehr its worth it to turn on teh wifi
5. steamy window - exactly as it sounds. places steam over the home screen, and you can draw on it, and beads of water fall off. then you can blow on the screen and it reapears.
and then of course I love beautiful widgets (still haven't updated, the old HTC replica is ftw!), pandora, weather channel, etc. I was using pandahome but it was laggy so I just wsitched back to OEM today. It was great how I could edit and use various skins/themes for various settings. I had the black stock background, a fallout 3 pipboy handle, another fallout 3 skin for the icons, and then use the oem settings for everything else.