If you're into yard sales, like me, you probably troll Craigslist towards the weekend to see what's going on. Craigslist does have a map, but it's not super thorough and some of those "sales" are really just a person selling one or two things.
But here's a new site: Gsalr.com (as in garage sale-r) with a map and a fair amount of listings -- more than Craigslist. There are a few in our area this weekend, plus some farther afield. It's a nice interface too.
Check it out! And if you're having one, put it on there!