Marathon Emblem Collector is the most complete source for Marathon emblems: what they look like, exactly how to get them, and whether they're still available.
MEC currently tracks 56 Marathon emblems
Welcome to the MEC alpha (v0.03.01.06)! Open TODOs:
-Finish adding all Marathon emblems - DONE!
-Finish adding all in-game emblem descriptions - DONE!
-Finish adding all emblem images - DONE!
-Verify all emblem data - HELP NEEDED!
-Finish Filter menu/clean up header menu -DONE!
-Finish adding emblems to the correct Filters -DONE?
-Fix mobile views - DONE!
-Add background cosmetics (one is on the site right now in super alpha, more to come!)