Some of them require access to the Injustice: Gods Among Us game on an iOS device (iPhone, iPad, etc.) so I've listed those separately.
This is currently a running list, so I'll be adding more as they're found. Please leave comments if you have found some that aren't on this list.
Here are the alternate costumes you can unlock, and how to unlock them.
- Boss Grundy – Complete Classic Battle Mode with any character
- Godfall Superman – Complete Story Mode with any character
- Lex Luthor Alternate – Complete S.T.A.R. Labs with at least one star
- New 52 Flash – Create a WB ID, or login to your WB account if you already have one
- Yellow Lantern Alternate – Win a ranked online match