Speaking of iPhoto, something I didn’t realize is that iPhoto has its own “Trash,” separate from the main Mac OS X “Trash.” This means that if you delete a photo, it doesn’t actually get deleted, it goes into the iPhoto Trash, and stays there until your empty the Trash (right click on the Trash in the sidebar and select “Empty Trash”).

Because I never realized this, every photo I’d ever deleted was hanging around in my iPhoto Trash, sucking up disk space — 7.5 GB of disk space as it turns out, as there were more than 6,000 photos there.