I've been using the Gardenate website for a little while now and find it great for helping me decide what to plant when.

The best part is they do a mobile app for your iPhone as well, this is one must have app for any iPhone owner who loves their gardening.

The Gardenate iPhone app puts their veggie gardening database in your pocket, localized to your climate zone.

It lists the best choices for what to plant in your garden right now. As well as a handy cal;culator on when your crop will be ready to harvest.