Some posters talked about henbit. Others talked about dead nettle. Had me totally confused, so I called on my friend Google. He knows everything. He said they are the same thing, just different names. Now I know what folks on here are talking about.
I'm going to have to disagree with that one. I think henbit is different than dead nettle. Dead nettle leaves are variegated, heart shaped. Henbit leaves look a little like parsley to me. I haven't seen any dead nettle around me but loads of henbit.
Here is a picture I found that compares.