Whenisgood.net. No sign-up required.
1. Create an event/meeting
2. Click the grid for all the times that are good for you and you will get a link to email to your invitees.
3. They see your proposed times and click on when they are free.
4. Visit your results page and see when everyone can meet.
Check out this how to video to see how simple this tool is to use. Click here.