GuideGuide - A free Photoshop plug-in for quickly adding guides
If you spent a lot of time adding guides in Photoshop e.g. for website or UI design, this plug-in could come in handy.
Website / Download (CS4, CS5 & CS6)
Dealing with grids in Photoshop is a pain With GuideGuide, it doesn't have to be. Pixel accurate columns, rows, midpoints, and baselines can be created based on your document or marquee with the click of a button. Frequently used guide sets can be saved for repeat use. Grids can use multiple types of measurements. Best of all itís free.