Tasker is, far and away, the best android app. No contest.

Not sure if anyone has mentioned it upthread or not.

It is the only paid app that I use, and it is well worth it. It replaces several paid and free apps worth of functionality, and it is still growing.

After using it, I wouldn't want any kind of phone without it.

Download directly from the developer (http://tasker.dinglisch.net/) or the market.

Note: I am not the developer or affiliated with him. It's just a plain awesome app.