App to Manage Photos on Linux - DigiKam
DigiKam is part of the standard repository for Fedora 20. It has lots of functions that really help when sorting through a lot of photos. The different views are great, and the image-editing part is fully featured. It can take a little time to work your way around the interface. However, it’s worth the effort and not only will your images be organised in a much more logical way, they may just look better as well.
Shotwell is the default photo manager in Ubuntu. It's much easier to use than digiKam, albeit not quite as feature-filled, but it should satisfy most casual users just fine. You can sort photos into events, add tags, edit them with a basic editor (crop, red eye, enhance, color correction)
Another simple photo manager is gthumb which runs under Gnome.