I'm using CeleTask.
It's very tiny and fast, which I like. It also displays a lot of important info (all optional).
I have it mapped to my right key using the program Right Key (configurable menu/program launcher).
I tried QTask too and didn't like it. The hot key thing is cool but I think it interferes with other programs (AEB, maybe Autokey lock).
See it here:
http://www.efficasoft.com/celetask/index.html