Could be lady's thumb, or Pennsylvania smartweed, or tufted knotweed. Anyone know for sure?

???

4/12/11: Turkish speedwell (thanks andrea, see comments). A delicate and hardy little groundcover.

Veronica liwanenis

See also September 23. Looks like, but isn't, rosemary. Even has the same tiny flowers. Has great fall color. [8-13: might be bog-rosemary per Andrea]

Jerusalem artichoke?

Most people, including me, wouldn't call these weeds. But they come up every year in the park next door and I've never been sure what they are. Deer don't eat them so I'd like to borrow a few plants for my yard in the hope they'll spread.

???

Spotted in my friend Shannon's front yard.

Yes, nutsedge. Specifically yellow nutsedge.

Cyperus esculentus.