Two sites offer some good electronic tools for planning a garden that let you play around with the arrangement of your flowers and vegetables and figure out when to start seeds, and when you should be able to harvest.
The first is a tool offered at Eat Close to Home - a good blog offering information on gardening, cooking, and other fun stuff. There you can drag and drop an assortment of vegetables into place. While the selection of things to grow is a bit limited, perhaps the best feature of this tool is the ability to click on a month and see how things will look at that time. This allows you to see how things expand or disappear (assuming you've harvested it to eat yourself versus a rabbit coming in to harvest for you) over the course of a season.
The second tool is a Kitchen Garden Planner offered by Gardener's Supply. This planner has options for pre-planned gardens with titles like Cook's Choice and Salsa and Tomato Sauce as well as a design your own feature. You choose the plants from a limited list and plunk them in place. A nice feature here is that the plants are listed below with seed-starting information and instructions for general care. You can also email it to yourself to keep track of different ideas.