Commonly known as Rattlesnake weed or Florida Betony, or properly known as Stachys floridana. This is one weed that has some benefits-- you can eat the roots! It grows about a foot tall and has white to pink flowers with purple markings. The stems are square, as is typical of members of the mint family.
The leafy parts of the plant can be made into a tea or steamed for a nutritious green. The flavor of the cooked leaves is not that great so you'll probably want to mix it with other greens.
There is a closely related plant (S. affinis) known as Crosnes or Chinese artichokes. Many botanists contend they are the same plant, but in organic and gourmet markets in New York, Crosnes sells for up to $5 per tuber-- the equivalent of $150 per pound!! They are served in some of the finest restaurants and priced accordingly.
Just think... those weeds in the back yard could be your next gourmet meal! Don't plant them on purpose though--they are invasive. Even if you harvest them regularly you won't get rid of them. The thin roots break off easily and any piece remaining in the soil will grow a new plant. I've been eating these for years and the number of them growing in my yard has not diminished in the least!
If you have this weed, try eating it. You might like it!