Synonyms and other taxonomic changes
includes Psithyrus, which is now considered a subgenus of Bombus - see links below
Numbers
42 species in the western Nearctic
24 species in the eastern Nearctic
17 species in the southern Nearctic
Nearctica lists 41 species plus 6 listed as Psithyrus
Identification
OVERVIEW OF SPECIES OF BOMBUS
Species affinis - Rusty Patched Bumble Bee
Species appositus - Bombus appositus
Species auricomus - Black and Gold Bumble Bee
Species bimaculatus - Twospotted Bumble Bee
Species borealis - Boreal Bumble Bee
Species californicus - Bombus californicus
Species citrinus - Lemon Cuckoo Bumble Bee
Species crotchii - Bombus crotchii
Species fernaldae - Fernald Cuckoo Bumble Bee
Species fervidus - Golden Northern Bumble Bee
Species fraternus - Southern Plains Bumble Bee
Species griseocollis - Brownbelted Bumble Bee
Species huntii - Bombus huntii
Species impatiens - Common Eastern Bumble Bee
Species insularis - Indiscriminate Cuckoo Bumble Bee
Species melanopygus - Bombus melanopygus
Species mixtus - Bombus mixtus
Species nevadensis - Bombus nevadensis
Species pensylvanicus - American Bumble Bee
Species perplexus - Confusing Bumble Bee
Species rufocinctus - Redbelted Bumble Bee
Species sitkensis - Bombus sitkensis
Species sonorus - Bombus sonorus
Species ternarius - Tricolored Bumble Bee
Species terricola - Yellowbanded Bumble Bee
Species vagans - Half-black Bumble Bee
Species vandykei - Bombus vandykei Image needed
Species vosnesenskii - Yellow-faced Bumble Bee 
Remarks
The following species used to belong to the genus Psithyrus, now considered a subgenus:
B. ashtoni Cresson 1864
B. citrinus Smith 1854
B. fernaldae Franklin 1911
B. insularis Smith 1861
B. suckleyi Greene 1860
B. variabilis Cresson 1872
Print References
Michener, Charles D. 2007. (2nd ed.) The Bees of the World. 1,016 pp. Johns Hopkins University Press. [advertised
here]
Internet References
Bombus Bumblebees of the World - British Museum of Natural History
The following range maps and species lists are all from the British Museum of Natural History website (the 2-letter abbreviations indicate subgenus):
common name references; PDF doc for 20 species of
Bombus (Committee on Common Names of Insects, Entomological Society of America)
discussion on subgenus Psithyrus plus list and distribution of world species (Natural History Museum, UK)
world bee genera plus synonyms and subgenera; list based on Michener, 2000 (Doug Yanega, U. of California at Riverside)