Usernames that we use in registering for an online service is generally described as our online identity. Most of them will appear on public pages for everyone to see. Some people use their real names for their online identity and some use aliases.
UsernameCheck is a great way to find out if the current username/identity that you are using online has been taken by others on other websites. What it does is search for the availability of your username across dozens of social media sites and services. It will tell you if the username that you seek is available or already taken.
The service is rather new and it only features popular social media sites and services. The site sometimes fails to load and there are some reports of inaccuracy to the results. For curiosity’s sake, it’s still a great way to find out if your username is being used by others and it’s also a great way to discover services that you aren’t familiar with.