I have sooooo many photos scattered on my computer. Not all are in iPhoto, or Lightroom, or Aperture, the latter on which development was discontinued by Apple.
Snapselect for OS X from MacPhun Software is a very clever and tremendously useful app just released on the Mac App Store. On sale for US$14.99 (40% off for a brief time) the app finds your photos in folders, iPhoto, Lightroom, Aperture, and even on SD cards that are plugged into your computer.
The app orders everything by date (timeline view), and using some patented technology it can find duplicates of photos. It can also show you full metadata and histograms for each image. RAW formats are supported, and you can go through the photos and reject those you don't want, which are sent to a special folder. Photos can be marked as favorites, and you can export photos directly from the app.
Although there are many cataloging apps out there that have a piece of the puzzle, Snapselect has the best feature set and speed I've seen and it can save you hours of hunting and sorting.