Anura is the tech behind the spectacular Frogatto & Friends. It is a fully-featured game engine, free for commercial and non-commercial use.
Note: For historic reasons – unlike most projects – Anura's main development branch is trunk as opposed to master. For example, to clone the repository, you might run git clone --branch trunk git@github.com:anura-engine/anura.git.
To compile Anura for Argentum Age (formerly Citadel), use the argentum-age branch. There are some issues on trunk which prevent AA from running there at the moment.