1974 D&D Original
1977 D&D Basic Set 1st revision
1977 AD&D
1981 D&D Basic Set 2nd revision
1983 D&D Basic Set 3rd revision
1989 AD&D 2nd Edition
1991 D&D Rules Cyclopedia
2000 D&D 3rd edition
2003 D&D v3.5
2008 D&D 4th edition
2012 D&D Next(announced, play tested and released as 5th Ed)
2014 D&D 5th edition
Two major editions in 20 years to four major editions in 12 years ... i guess that's why it feels like a lot of new editions. And they are still yet to embrace online let alone mobile technology.