Like the other three titles, the open-source Aquaria code contains just the game engine and not the game's data/artwork, so you will still be stuck needing to purchase the game if you're interested. Aquaria is a game that Ryan Gordon had ported to Mac OS X and Linux last year.
The source-code to the Aquaria game can be found at Icculus.org. All the details about the open-source Aquaria can be found on the Wolfire blog.
Aquaria is described as "an underwater fantasy world to explore" with "magic and combat at a mouse click" where one can also create their own worlds with a level editor, which has also been opened up.