Whatsonwhen.com
There are few surprises with this site, which does what the name suggests. If you want to know when a particular event takes place you can look it up by name. Otherwise search by location, date and type of event (art, music, sport, festivals, family) and it will tell you what's on and pinpoint it on a map. It does come up with some permanent attractions too, which can be good or bad depending on what you're after. My one quibble is that, if you do an open search, cafés, restaurants and other businesses that have advertised with the site come up, irritatingly, within the listings.
Like that? Try www.wherewillwego.com