Looks like blight to me, but could be something else. Are you doing anything for blight? Spraying or using blight resistant varieties?

That's just early blight, very typical in early summer on the lowest leaves. I usually clip them off to slow the spread of the fungus, which rarely goes higher than 18 inches because it needs damp leaf surfaces to spread.

Agree Early Blight. Remove all affected foliage and dispose away from the garden and begin regular fungicide spray problem if you wish.

Lots of similar discussions right now over on the Growing Tomatoes forum.

Dave