The latest version.
KeeperRL alpha13 for Windows, Linux and Mac, released: Jan 29th 2015. The commercial version is exactly the same as the free one, except it's with graphics and music.
This is an alpha release of the game. If you decide to purchase, you will have access to all future versions, including the final release. But the game is not completed yet, and some features might not be working. Please keep it in mind when purchasing.
Previous versions are available here.
Another way to pay for the game is donating the same amount to Wildlife Conservation Society (or any other wildlife charity). Just forward me the confirmation email. So far $353 has been donated.
The source code of KeeperRL can be found here. The repository contains the latest development code. You can download it with the following command.
git clone https://github.com/miki151/keeperrl.git
The source distribution doesn't contain music and graphical tiles. The game works fine without them in ASCII mode.