Use this code NTQ-Q8ZDZ-YVNJV on Steam to get Evolve alpha on your account for free. Even though you will see that the key is invalid or used the game will be added to your account.
Update: You need to have XCOM Enemy Unknown in your library for it to work, at least that’s the rumor on the net. It worked for me.
If this code for some reason doesn’t work try those: