I really like Doodle for scheduling. I remember scheduling practices back in 2009 with dozens of emails back and forth to figure out when everyone was available. Being able to send something out and have people indicate which days work is just so much better.

You can use Doodle for things aside from scheduling, though, any time you want to collect yes/no from lots of people. For example, over the holidays my family used a Doodle poll to create a shared grid of which days people would be in town:

Each person filled in their own information, and then we used it for planning dinner, housing, etc.

I think Joel Spolsky is mostly right that "the great horizontal killer applications are actually just fancy data structures" and Doodle is a shared two dimensional array of booleans.