Have you considered allysum? I know it isn't a perenial, but it does reseed itself quite easily, and comes in a variety of colors.
There is also viola. I have a variety in my garden, I believe its "King Edward." It is a velvety dark purple with bright yellow centers. It spreads quickly, (its even spilling into the lawn!) but I am not sure if this is from reseeding or root runners. Either way it's beautful. I have mine growing in full sun with no problems, even during dry, hot weather. They started blooming in the spring and are still going strong.

Phlox. There are both tall and short varieties.
Catmint is a great plant.
There are some lavenders that might work.

Yarrow is great if not to hot. Yellows are great.
Mike