If you're heavy into social media sites it can be a challenge to keep all your contacts aware of what sites you're on and what your profile name is at each one. Sure, you can try to get the same profile name everywhere but that's not always possible.
Enter Card.ly. Here's the official description: Card.ly is a site thats main purpose is to allow you to have your own mini business card profile, without needing to have any design or programming skill. Note the emphasis on business, but what I find compelling about the service is that it's a quick and easy way to gather all your social networking profiles into one place. Pick a service, enter your profile name and hit the Add button. Once you've entered your profiles, choose a theme and you're done. You can also add bio information, company contact information, phone numbers, an RSS feed, IM names and there are even freeform fields available. Then pick a widget (though these are pretty basic) and embed it wherever you post frequently.
The downside is that none of this information is confirmed in any way, so you could list someone else's social media profile as your own (either intentionally or via mis-typing). Card.ly seems to be a work-in-progress though, and the developer is actively soliciting suggestions to make the service better; hopefully a way to validate profiles is in the works.
A basica Card.ly account is free, though there is a Premium option that (according to the site, I didn't test this) gets you access to more themes, unlimited RSS feeds and a bunch of other perks and improvements. Premium access is $24.99/year or $2.99/month.
My still-rudimentary card.ly page