If you're particular about your username and want to have a unified personal username or company identity across all the social networks and services you'll be using, KnowEm makes checking availability simple.
Like previously reviewed NameChk KnowEm checks to see if usernames are available. While KnowEm is distinctly geared towards business users who are trying to control their brand name — they have paid packages where they will register and maintain your brand on X number of social sites for Y number of dollars — it's just as useful for a casual user and has more than twice as many entries as Namechk.
Plug in the username you want to use and hit "Check This Category" for each category you wish to check. You can check sites in categories like blogging, bookmarking, business, community, design, entertainment, health, information, microblogging, music, news and more. Even if you're not really looking for a unified username the massive chart of social networks is interesting to browse if you're looking for somewhere new to hang out online. Using KnowEm to search for username availability is free; the only cost is incurred if you pay KnowEm to register and tend your username on multiple sites.
Have a favourite tool for checking or managing your identity online? Let's hear about it in the comments.