Black Eyed Stella is another reliably hardy plant here in zone 3. Like most of the other comments, no rebloom here (but hey, zone 3). But it puts out a lot of flowers during the time that it's blooming - flowers just about cover it, you can see them from across the garden.
It does resemble Sweet Seneca Kisses (also early) and Leprechaun Eyes (midseason), both of which also do well here, so I can totally forgive its lack of rebloom.
Sweet Seneca Kisses
Leprechaun Eyes
