Chickweed Use a herbicide with triclopyr in it. Weed B Gone for Chickweed and Oxalis will do it for you.

I think sedge too. Pull one and look at the main stem. Is is triangular? Growing leaves in roughly three different directions? (grasses grow leaves opposite one another, sedges grow them roughly 120 degrees from each other, hence the triangular stems.) Are the leaves shiny and waxy? If so it is almost without a doubt nutsedge. Put more simply, if the stem is triangular it is a sedge, and not a grass, and the only thing you need to figure out is what sedge, exactly, it is. But still if the stem is triangular and it is growing in your lawn it is probably some variety of nutsedge. Which is a problem, because these weeds grow underground storage systems (nuts - hence the name) to help them rejuvenate after you pull them. So to get rid of them you have to either pull them like every other day for a year, or apply a product like Sedgehammer. MSMA also has an effect on them, but it is relatively mild and not nearly as effective as the more expensive and harder to find sedgehammer.

Would it be possible to get a bigger, higher-resolution picture of it? I'm having trouble even telling whether it's a weed grass or a herbaceous plant from that little photo...

The grass looks very much like crabgrass, rip it out before it seeds! It'll take some time to get rid of it!