Windows only: If you find yourself stuck with multiple copies of the same Photoshop project, free utility ComparePSD might help - it quickly compares PSD files layer-by-layer, effect-by-effect to find the one you're looking for.
If you're lucky, you've learned the lesson "save early and save often" — and if you're really lucky, you've learned to save a new copy each time, so if you make a mistake you can go back to an earlier draft. However, that creates a whole new problem when you don't know which file is which — and no one wants to open each one up in Photoshop and look through the effects in each file.
ComparePSD lays these files out side by side and lets you compare the files layer by layer, so you can notice effects differences easier — with thumbnails to make it all the easier. It also integrates with the Windows Explorer context menu, letting you select files to compare straight from their folder. Plus, as of today, it's free — and it certainly beats waiting for Photoshop to load.
ComparePSD is a free download, Windows only (although they are working on a Mac version as well).