Unity Technologies, the company that developed the award winning game engine Unity, announced at GDC 2012 that the iOS and Android Game Builder version are available for free forever until April 8th.
Currently valued at $800, the two smartphone versions allow users to create games straight out of the box, with a range of default assets supplied, as well as unique controls and in-depth manuals that are easy to understand. There is a wide range of games currently on the iOS and Android market that utilise the Unity engine, including Chop Chop Ninja 2, Shadowgun: Deadzone and SKAR: The Hammer Returns.
So head over to the Unity Store
where you can get the Unity client as well as the iOS and Android Game Builders license, absolutely free of charge forever. Hurry though, it's only free until April 8th, 2012!
Edit - To clear any confusion, the client and iOS/Android licenses are absolutely free of charge as long as you download before April 8th, 2012.