Jamie posted a review of Linux a while back, and in it he mentioned the Shotwell photo organiser. I was revved up to give it a go, as I miss a decent photo organiser after a productive time with the proprietary and lovely-to-use ACDsee.
Alas, I am now shot of Shotwell, as it does what F-Spot Photo Manager and iPhoto do: imports images into it’s own image library, rather than dealing with existing images and existing folder structures. I find this a problem because it’s a) slow and b) duplicates images.
The good news is that I had a look at gThumb for Linux, and this is far simpler and more in keeping with the ACDSee method (although it still doesn't have thumbnail zoom functionality, and the shortcut rotation tools aren’t very… polished).
Most of the time I just use the GNOME Nautilus file manager to manage the few photos that I take nowadays.