If you use Photoshop to do any website or UI design, you'll know how annoying and time consuming Photoshop's guides tool is. You're made to painfully add guides one by one which is just plain annoying.
GuideGuide is an absolutely brilliant tool which makes making guides real easy and can be done with one click. It's a completely free Photoshop plugin which enables you to add guides however you like. You can add whole guides at once or just one guide. You can also save the guides to use later in other Photoshop files.
Seriously, this is so useful. I'm annoyed I didn't discover this tool earlier instead of wasting so much time.