Could be sand burrs, AKA sticker burrs. When I first moved to Texas in the springtime thirty years ago my yard had a huge patch of beautiful "clover." I carefully mowed around it until early summer, when the lush green area turned into a torturous mass of painful little stickers that not only drove serious pain into bare feet, but stuck to shoelaces, clothing, and my poor dog's feet also. If they are sand burrs, I've found only two solutions without resorting to poisoning my yard.
1. After a good soaking rain, get on your hands and knees and pull out as much as you can. Repeat as necessary.
2. Set your lawnmower as low as possible and run over the stuff every time it starts to flower.
good luck!

Or it might be Oxalis (Yellow Woodsorrel). In that case I use my handy dandy little yellow spray bottle (that had fake-butter spray in it) fill it with full strength broad leaf weed killer and very carefully spray a little on the leaves of the woodsorrel being careful not to get it on anything that I don't want to die. If a drop gets on something good I just pull that leaf off.

I carry the bottle in my pocketed gardening belt. It comes in handy for a lot of things, like for killing weeds that are hard to pull up -- for instance those little hackberry trees that the birds plant. In that case I snip the tree off near the bottom and spray it on the little stump.

Here is a link that might be useful:

Could it be black medic? They look like clovers. They have tiny yellow flowers that looks like clovers as well. It was a huge problem last year in my subdivision but so far nothing is happening. The entire neighborhood park last year was covered in black medic and apparently improved the soil so much that bermuda has spread out and thickened up most of the park. It's the best looking I've seen since I moved here.

The point? Mother nature has a weird way of improving soil... Weeds are what makes soil better in the long run.

Thank you all, after Identifying the little weed, I found this

http://www.au.gardenweb.com/forums/load/ozgard/msg0721021131949.html

Looks like I will have to get on my hands and knees and hope for the best.

Since you mention it Lou, black medic might be what comes up around here. I don't remember the clover looking plant with little yellow flowers having bulbs, it just had a little root, which is good -- that would make it easier to get rid of, but I don't have any growing right now to compare.

Oh, jeez, tell me the pink flowering oxalis isn't bad. I was digging up other plants from a ladies yard that she was getting rid of and saw those pretty little clover clumps with the pink flowers and put them in the front of my new flower bed for a low growing filler. They looked cute but ended up getting covered by everthing else when it grew up. Now that everything is dormant I can see them again. Should I get rid of them? They are bulbs.

No, the pink ones are wonderful! I've never had a problem with them. My are green now and are early spring bloomers.

Aw... Some "weeds" are really just flowers without good public relations staff.

Pink oxalis are awesome, and yes, they'll spread, but I've never had issues with them choking out other flowers in the area, and they're one of the first things to bloom in spring in my garden.

Oxalis aren't true clover, but true clover are legumes, which help fix the nitrogen in the soil, making it accessible to other plants (and thereby building it up). Clover isn't a bad plant, either.

Oh good. They are rather pretty. I had intended to put them by themselves in a small bed around my crepe myrtle but it was so loaded with those pink weed flowers I couldn't get rid of them all. When I made the new bed it was pretty bare at first and I put a few cuttings of mona lavender in there and they went crazy and buried the pink oxalis, lol. Then I planted a flowering sage of some sort in the back for height that blooms purple and it was really pretty but got huge and leaned way over sideways. I think it was growing toward the morning sun. It has died back so I think I will cut it back and move it to a spot with more room. My poor flower beds! I am such a plunker, lol.

My oxalis must be from Three Mile Island or something because those suckers are totally invasive. I have spent the last two springs trying to get rid of them and they come back stronger than ever. They are the more magenta pink variety. The previous owners of our house planted lots of super invasive species and I curse them every time I work on the gardens. Oxalis, English ivy, Virginia creeper, the x&@#($# perennial morning glory. What were they thinking? I garden organically so a chemical herbicide is out of the question. This spring, despite a heavy application of pre-emergent corn gluten, they are back. My current plan is to cover them with layers of cardboard and build up the soil. I've tried digging them out, but any little bulb left multiplies like crazy. We are going to sacrifice the other plants in the bed because these species are just so overwhelming.