Friend Connect

I've been mucking around with Friend Connect on and off for a few months.
It finally launched, which is nice. There's a video, and links to sign up if you want to play with it.

It's a great tool, letting any site easily add a social element to it, with sites like Mashable already hooked up, which is pretty cool.

It also supports custom gadgets, built using the OpenSocial spec, where the owner of the page is the site itself. You can do some pretty interesting things, I reckon, with a bit of thought.

For example, a games site could easily add a gadget which let people share their gamercards without only pasting some code, just like a YouTube video (and a bit of programming for the gadget itself).