Over at Gardener's Supply, they have a pretty good garden planner that allows you to size the bed to your desired dimensions and gives you planting information such as spacing, when to plant, etc.
I'm not fond of most of the planners I've seen; they tend to have an ugly interface or be clunky, but this one is neither, and the only annoying thing is the chime when you complete an action like dragging and dropping veggies into the plan. It sounds way more enthusiastic about growing Brussels sprouts than I'd ever be.
Allows you to print out your plan, too.
If you only have a small space to work with, or if you want to try intensive gardening, it's also good for that; on a large scale, I think you might have issues with multiple beds as it only allows one bed at a time. It does have the advantage of being able to rename things once they're dropped onto the plan; very useful for varietal information, or for when you use a generic plant to represent something not in their list of basic plants, like renaming a parsley to 'mint'.