Start an Application in the Background in OSX at Login

It's easy enough to set an application to start at login with a Control-Click of the application in the dock and checking "Open at Login" under Options, but if you want that application to start unobtrusively at login you'll need to go into System Preferences:

System Preferences > Accounts > [ Your Profile ] > Login Items

From here, check the box to "hide" the application.