Designing grid-based layouts in Photoshop is usually a headache as it takes time to calculate and position many vertical + horizontal guides.
GuideGuide is a free Photoshop plugin which simplifies creating grids in Photoshop with only a few clicks.
After defining a few variables (number of rows/columns, gutter width, etc.), it instantly generates the grid using Photoshop guides. And, for any active selections in the document, the plugin sets the grid within the limits of the selection.
Also, any frequently-used grid settings can be saved for later use.
P.S. Smashing Magazine has a very nice article on the usage details of the plugin.