dannybee,
I'd suggest you do an internet search on this topic - you might be surprised at all the available information there is on just this one topic! Vitex is good and there are groups, even here at GardenWeb, that deal with swapping seeds or trading honey for seeds (which I've done before).
But ultimately, I feel (some my disagree), that planting specifically for your bees is a somewhat fruitless endeavor due to the shear amount of land and planting volume you'd have to do, to really provide a substantial nectar source for them. [But that has never stopped me from planting at least some nectar sources for them].

I planted a small patch of buckwheat last year. It grows fast, you can do two or more plantings, the bees like it and the flowers are pretty which makes for some good photo opportunities.

Buckwheat is an excellent "green manure" crop as well as being good bee snacks.
It puts a lot of nitrogen back into the soil & makes for a good soil conditioner.
Folks that have gardens (and the room) may want to consider doing a crop rotation every couple-few years with buckwheat & plow it under after it's run it's course.

Wildflowers are always easy and a joy to have around. My bees adore "Snow in the Mountain" and the Sunflowers, both of which bloom very late and stay in bloom a very long time.

I am very lucky to live on a dead end street with no houses on one side because of the river that goes by. The neighborhood is a bounty with wild sunflowers, a small field of wild thyme, sweet white clover,japanese knotweed,goldenrod,milkweed, sumac, basswood trees,weeping cherry,some fruit trees, and the always welcome dandelions. Throughout the season there is always something providing nectar or pollen.