My favourites are “my punch bowl” for A-Z free event organization, and “eventbrite” which is very professional and free for free events. Then a commission for paid ones. “aimado” is very good too.

Then you can also organise events using outlook or iCal.

Social networking tools such as Facebook allow you also to create events and manage attendance.

A very simple tool to agree on a date or on what to bring or whatever is doodle.

The best professional one cvent.