Looks like Henbit (Lamium amplexicaule), a member of the mint family.
The best time to control Henbit (non-organic) is in the fall with a
postemergence broadleaf control product.
An organic approach would be to pull these weeds now, and mow high
(2.5-3 inches) the rest of the year... pulling the henbit when you see
it. The taller grass should smother the henbit seedlings.
I will be composing an article about Henbit on our website at the
following page: http://www.stockseed.com/weeds_henbit.asp ... look for
it in a few days.
Update: I have posted the information for Henbit to
http://www.stockseed.com/weeds_product_display.asp?pid 3 instead of
the link previously posted.