Valve responds with a fix
There are reports of some Steam users experiencing weirdness with their libraries today. Specifically, Valve games appear to be swapped for some players so, for example, Left 4 Dead 2 might show up as Team Fortress 2. I haven't seen any of these issues first hand, but if you have encountered them, Valve has instructions on what to do next:
Please click the following link and confirm the prompt within your client -- that should refresh configuration of your client.
NOTE: If you have multiple installation folders for games completing this process will remove them. Perform the following steps to add extra installation folders in your Steam client:
1. Open Steam
2. Click Steam > Settings (Preferences on Mac)
3. Navigate to the 'Downloads' tab
4. Click the 'Steam Library Folders' button
5. Click on 'Add Library Folder' and add the directory that contains your games
6. Click 'OK'
7. Restart Steam
Library Insanity [reddit]
Weird issue with games being fused/swapped in Steam [Steam]