You may not have noticed this plant before, but once you can identify it you'll start to see it everywhere. Self-heal is a perennial that can actually compete (somewhat) with lawn grass, especially where there is moderate foot traffic. You'll frequently see it on the edges of trails through moist, forested areas and near creeks. If you have disturbed areas around your home that get some irrigation (and therefore fill up with a whole assortment of weeds), try self-heal. And, true to its name, this plant has a long history of medicinal use.
 
The flower is a tight oblong bundle with purple petals bursting from the sides subtended by dark maroon bracts. Even after flowering, self-heal has an interesting cone-like appearance in various pastel shades of pink and yellow.
 
The plant itself is low-growing and will reseed readily on open ground. While it prefers partial shade, you can get away with full sun if there is sufficient moisture. Despite liking water, this plant doesn't like to be flooded for long periods of time, so plant where it can drain.

I highly recommend annuals if you are trying to quickly fill a space before the weeds take over, and this is my only good shade-tolerant one so far. Plant along with large-leaf avens and Canada violet for fast-growing ground coverage.
 
 
 
Photos of plant in bloom:
Robert Flogaus-Faust, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
Bjoertvedt, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons