[avirambm]

http://doodle.com

[timtas]

Shameless plug for my app. http://slotted.co

Many people use it for one-on-one meetings like office hours. But you can make any kind of signup sheet you can imagine. Very geek friendly UI.

[rattray]

http://when2meet.com as well

[reledi]

I've been looking for a Doodle replacement. This looks promising.