What do I do with Kingdom of Amalur's Steam Key?

Steam provides you with the cd key for games you have just bought from it - most notably for those that are neither created by valve nor published exclusively on steam. This is in case you ever want to register it on a secondary digital distribution platform (This is sometimes required for the game to launch as in the case of Games for Windows Live). You may never need the key, but it's useful to have for reference. It's the proof that you have bought a unique copy of the game. No need to worry though, Steam will keep the information for you.

Edit - I might add that you can still access your CD Key for those games that ARE developed by Valve or published on exclusively on Steam - it just doesn't pop up every time to launch the game.

You can enter the key into EA Origin and it will be registered to their download service :)