Shortly after Facebook announced the Chat Heads feature of Facebook Home, the folks behind ParanoidAndroid started expanding that idea into a new take on multitasking. HALO was the result, but it was exclusive to ParanoidAndroid. I say was, because HALO has just been open sourced.
The code has been added to a Github for other ROM developers and curious users to play around with. HALO is still in beta, but the feature set has come along quite well in recent weeks.