It is easier to maintain one username across all online profiles, so it is better to register your username early on web apps otherwise someone will grab your username. UsernameCheck.com helps you to check the availability of username at various popular web 2.0 sites. Most of these sites are user generated or popular messaging tools.
Enter your desired username in the input box, it will check each and every service it supports for that name availability, immediately it will show you whether your username is available or not. If it is available it will give you a line to sign-up.
This is a simple service but useful to grab that unique username of yours. Go and check your username at various sites and grab them if they are available.