Steven, what a great thread!
I've had very good luck with almost any of the rudbeckia (once they get started, they self-sow nicely). Some favorites; Irish Eyes, Indian Summer, Cherokee Sunset.
Also, I grew Scabiosa blackamoor last year, and it was awesome. Took a little while to get going, but bloomed until a hard frost, and the color was incredible.
Zinnias are also easy to start from seed, even if direct-sowing. I've grown Wowie Zowie (a real butterfly magnet), Green Envy, Purple Prince, and Come and Cut Again. They help keep my vase of cut flowers filled for a good portion of the summer. I saved seed from the Wowie Zowie and replanted last year--they didn't come quite true, but very close, so lots of the bright yellow/orange color that is so attractive to butterflies.
Snapdragons are also easy from seed, and will re-seed easily without being invasive. I grow the taller Rocket Mix and get a wide range of colors. They also bloom until frost, and just really are so hardy.
