Wizards is getting the word out that a major goal of the new edition is to reunite the D&D fan base. I figure it would be useful to list all the distinct versions of D&D that are out there. This list focuses on those games released by TSR/Wizards.
Chainmail Fantasy Supplement in 1971
Original Dungeons & Dragons published in 1974
Supplement I Greyhawk published in 1975
added rules made Dungeons & Dragon into a form we recognize today.
Basic Dungeons & Dragons by Holmes in 1977
Advanced Dungeons & Dragons in 1977-1979
Basic/Expert Dungeons & Dragons by Moldavy/Cook in 1981
Basic/Expert/Companion/Master/Immortal Dungeons & Dragons by Mentzer in 1983.
Unearthed Arcana for AD&D in 1985, Some say that this made AD&D 1.5
Advanced Dungeons & Dragons 2nd Edition in 1989
Rules Cyclopedia for Dungeons & Dragons in 1991, A one book compilation of Mentzer's BECM D&D, some consider this the definitive version of the original Dungeons & Dragons line.
Skills & Powers for AD&D 2nd edition. Some say that this made a AD&D 2.5
Dungeons & Dragons 3.0 in 2000
Dungeons & Dragons 3.5 in 2003
Dungeons & Dragons 4.0 in 2008
Dungeons & Dragons Essentials in 2010. Some say that this made a D&D 4.5
Evreaux's great megadungeon player intro
40 minutes ago