Meetup- for regular, repeating special interest group events.
Facebook- birthday parties to invite people you know. Can import events from meetup with an app. The events on facebook tend to be undiscoverable unless your friend is the organizer. To really promote events on facebook, you need to friend lots and lots of people, which really makes facebook less useful as platform for letting friends and family know what you are up to in your “semi-personal” life.
Twtvite/Tweetups- To invite random strangers to hang out with you at a bar and talk about twitter. Well, I’m being cynical. I thinks twitter driven events are very promising. Twitter is already proven a good fit for one type of event, the conference. Maybe it will scale down for small events if people tried it.
Myspace- for promoting and finding music concerts. You can list small non music events, but what is the point?
Craigslist- for finding people to join a club (groups), but the event section is actually better for listing paid events like cooking or dance classes.
Evite – for inviting everyone at your college or office to an awesome party at you house that will possibly wake up the neighbors.
Traditional newspaper- for huge events, like a county fair.
Special interest sites, e.g. Nerd Dinner or GoodReads. These tend to have sparse user bases so you might have a hard time finding attendees from these places, but they’re good options for RSVP engines if you already have a group.