Here are a couple of grid templates for InDesign (around 450kb each). They are only for 8.5 x 11 inch pages and clearly for print layout, but you could use these as a starting point for developing a web-based grid system as well. Just change the units and increments to your needs and design it along similar principles as this (ie: proportional widths, gutters, etc.)
A 6 column grid: http://www.christopherhethrington.com/downloads/Grid_8.5×11_6c.indd.zip
A 12 column grid: http://www.christopherhethrington.com/downloads/Grid_8.5×11_12c.indd.zip