Some of the Florida beekeepers are advocating Crepe Myrtle as a bee plant.  I believe that Crepe Myrtle supplies only pollen to the bees. So, the myrtle will help the hive produce brood.  But it will not directly increase the honey crop, only indirectly by allowing the hive to raise more bees. 
I have tried to follow my bees to find what plants they are harvesting. And I have looked around my hives to see where the bees are harvesting. I have been fairly incomplete and inconclusive.  But it is surprising that you will not find them on anything unless there are large numbers of blossoms in a small area.  The bees definitely follow a well-defined pattern.  If you look carefully at the front of the hive, you will see that the bees go off in several definite directions.  They have standard approach and takeoff patterns corresponding, I think, to particular nectar and pollen sources.  And, the only places that I find the bees are places where there are large numbers of flowers in a small place.  A patch of clover 20 feet in diameter 1/4 mile from the hive is not likely to attract any of my bees.  I have only followed the bees to a flowering plant successfully twice, a blackberry patch in Sperry, and maple trees this spring near my house.  Usually, they fly up so high that I loose track of them. 
The big sources for honey seem to be various clovers, (white clover, alfalfa, vetch, lespedeza) borage, tansy, and some herbs. 
There are several sources on the web listing plants for bees:

http://www.apis.demon.co.uk/beekeeping/plants.html
does not list the clovers.  But it does mention most other common plants.  And it rates them qualitatively relative to how good they are for the bees.

http://beekeeper.topcities.com/articles/beeplants.html
gives the concentration of sugar in the nectar of various flowers.

http://www.aces.edu/dept/extcomm/publications/anr/anr-351/anr-351.htm
Soybeans and cotton are supposed to be good honey producers.  This also has a list of plants that are nectar and or pollen producers. 

 

http://www.rhs.org.uk/research/biodiversity/plantsforbees.asp

More plants for bees from the UK-

http://www.lal.ufl.edu/Other/beeplants/Page.html
Gives photographs of blossoms that are good for bees. 

 http://www.seed-man.com/bees.htm Sells seeds for bee plants

http://www.ces.ncsu.edu/depts/ent/notes/Beekeeping/bee2b.html
Landscape planting for bees

http://www.xerces.org/poll/plantsforbees.htm
-Native plants for bees- >