If your tomatoes have something like this on their leaves.... you have Early Blight. It affects tomatoes (this is Juliet) and peppers many times in our area when we do not keep our plants warm enough on cool or cold nights. (this photo taken 3-29-09)
This stuff works. I will show you a photo of the same plant (one week later) tomorrow.
REMINDER!!! Cool weather coming in tomorrow. Your tomato and pepper plants have been basking in the warm weather for the last few days and will not respond well to the cool (cold) night. Get them under some warm cover.
If you forget (or do not believe me when I say the "sky is falling") you can buy this product at The Antique Rose Emporium.