Probably Oxalis. Ortho has a chickweed, clover and oxalis killer that you can use on lawns.

Looks like Oxalis articulata. Not a true clover, despite the trefoil leaves. Clovers are Trifolium sp.

These can't survive *as well* where the grass is kept mowed short.

Its actually better for your grass than store bought feritilizers! check out some articles online. it wont choke grass out, and the nutrients it provides support healthy soil.

Panda, I think you are talking about Trifolium, not Oxalis. The plant asked-about 8 yrs ago was a bulb-forming Oxalis.