What is the best alternative to Adobe Lightroom on Linux/Ubuntu?
I quite like F-Spot + UFRAW for organising photos and processing them but I also use Darktable for when I get bored of UFRAW.
I use Darktable too. If your camera is supported RawTherapee will be an option as well.
I have been quite happy with using Bibble 5.
While it is a commercial program, I find it outstrips all open-source solutions in both usability and speed - at least for my uses. And they have quite good Linux support, including .deb-packages and simultaneous releases on all platforms.
Darktable all the way. Extremely versatile and well-thought-out RAW workflow tool. It's a new-ish project, but you'd never know it from the features and stability. There's an official release, as well as PPAs available for those of us who like the latest and greatest (and don't mind the odd hiccup, though those have been rare for me).
Be sure to watch the tutorial videos on their Sourceforge page... many features and functions are not immediately obvious to first-time users.
I've also used RawTherapee to great effect, but at the moment, Darktable beats it hands-down.