I have created a list for Michigan many years ago and never quite finished it with links to photos etc.
Early May: apples, cherries were 2-3 weeks ahead of usual this year (2010).
15: Blueberry (Vaccinium spp, Ericaceae) were also 2 weeks ahead this year.
26: Black locust (Robinia pseudoacacia, Fabaceae) has been blooming for about one week. Very good flowers this year. Not sure if it is “flowing” with nectar.
Tulip popular (Liriodendron tulipifera, Magnoliaceae) is in full bloom at MSU campus today. Did not see bees but I could use my finger to touch the dried up nectar as there were so much there. It tasted sweet. Zach
27: I went back today and took some pics of the tulip poplar tree, but about 15 min, no honey bees.
White Dutch clovers (Trifolium repens, Fabaceae) have been blooming for at least 2 weeks.
Iris blooming everywhere, most iris do not attract bees, but I did see bees foraging on them, once in Beijing, China, and once in Okemos, Michigan.
here is a photo to prove my point
13: Elderberry (Sambucus canadensis) begins to bloom in south central Michigan. I am unsure as to the benefits of this for honey bees. (posted by Steve Tilmann)
Please report which bee plants are blooming in your area (with both common name, Latin name, if possible), if it is not mentioned here.