How about petunias? They'd be gorgeous in the planters you describe. Maybe a deep purple picotee, or the new surfinia or wave petunias. Or you could plant verbena, maybe purple (can you tell I love purple?) or red temari. My brain is going to sleep. That's all for now.

Judith

Ivy geraniums would be pretty also. They cascade nicely and come in many colors. They can take the heat. Eight inches is not a huge depth - the geraniums can take the heat. You may want to experiment with a variety of plants this summer and see which ones like your locations.

I like dwarf lantana in containers, they come in different colors and bloom all summer into fall and requires almost no watering. The other posters have good suggestions too, geraniums seem humdrum, but they bloom like crazy and come in many different varieties (zonal, etc).

Here is a link that might be useful: lantana

I'll second verbena. It was suggested to me by a friend as an excellent container plant, and it didn't disappoint, blooming all summer long, very vigorously. The kind I had was the "Superbena" type from Proven Winners. This year I'm trying to grow a delicate orange verbena from seed.

Here is a link that might be useful: Superbena