I'm still not comfortable sharing my location with strangers, and my friends are all really low-tech, so FourSquare has never appealed to me. But if you are an avid FourSquare user, you'll like this one. Geo startup, Movity, built Weeplaces over the weekend. It's a simple idea to visualize your movements via FourSquare check-ins.
Blue circles show where you have visited. Places you have visited are represented with larger circles. A time series chart on the bottom shows your relative volume of check-ins over the days. Let the animation play, and a yellow line shows up connecting your current location to the previous. So what you get is something that sort of shows your movements.
Shaded regions on the map also become more opaque as you visit them, which is a nice touch, I think. You can see where you've been, and more importantly, places you still haven't checked out. This feature only seems to be available for certain regions though.
Also, since you probably don't check-in everywhere, it's going to be missing some things (especially if you're not very active), but for others, you should get a kick out of it. Try it out for yourself.
Here's a video of one user's check-ins.
I was going to say the style reminded me a lot of Sha Hwang's NYC ridership map, and then I realized he's a member of the Movity team. Oh. I suspect he had something to do with this.
Another update: Privacy stuff noted and fixed.