LightZone | Open-source digital darkroom software for Windows/Mac/Linux. Just remembered, it's now free. And because it's Java, it's available on multiple platforms. During my experiments back then, it was quite a cool alternative for Linux. Specifically, the contrast control through a digital zone system is quite cool, and the program saves JPEG previews and the editing steps directly in the previews, so you can easily exchange them between computers. Attention: currently, Lightzone still requires a license key. The whole licensing stuff still needs to be removed, but the source is already on Github, it's just a matter of time.