That's the Septagon emblem, not a Cog btw. If you've played H3 or ODST they're unlocked too, if you've played Marathon:Durandal on XBLA as your last 5 games that's another one. If you played Halo CE or H2 on PC and have your disc key that's another one.
Bungie pro nets you another (looks like either an AR or BR, can't remember which) while it's still available, and if you unlock 5 of the ones I mentioned I think that unlocks a Helmet. The Bungie one isn't available anymore, and afaik- the star can't be unlocked
unless you have the Halo App on your Iphone. Although, that may not be available anymore either.