Also called Ohio Horsemint, Downy Wood Mint features tight clusters of blue-purple flowers with a long bloom time.  It kind of looks like Spotted Bee Balm, the way the blooms stack along the stem, one on top of the other.
 
It is clump forming and easy to grow, preferring dry soils but adaptable to moist as long as it's well drained.  Even though it's in the mint family, it's not a big spreader.
 
A great addition to any garden setting.