After making their million dollars in the charity-driven Humble Indie Bundle, all four of the indie game developers that promised to release their source code have prepped and released that source.
Bundle organizers Wolfire have the relevant download info for all four of the games, at the following links: their own Lugaru HD, Cryptic Sea's Gish, Frictional's Penumbra and Bit Blot's surprise addition, Aquaria.
The Humble Indie Bundle [Wolfire]