Shotwell is a very easy to use photo management application.
It provides basic image manipulation commands (such as crop, red-eye reduciton etc.) and also some more advanced ones (export to Facebook, tagging).
The best feature of all: it's fast! Startup is almost instantaneous, switching pictures is quick.
It's already quite usable despite being in such an early development phase.
Stay tuned and watch it to grow into your favourite more!