It was kind of asked about here, so I figured I'd type up what I recall. Might be fun to fill in all the missing information.
2008-2009: Haven and Hearth launches for the first time.
2009: world 2 is launched
late 2009-early 2010: world 3 is launched
December 2010: world 4 is announced and launched due to hardware malfunctions. It lasts about six weeks due to a glaring bug in the new mining system
Jan-Feb 2011: world 5 is launched with the new learning point system, curiosities, to much consternation of many of the existing players. Many flames had to be quelled.
Winter 2011: Salem is announced to the HnH community to be published by Paradox. Work on Haven greatly slows
July-Aug 2011: world 6 is announced and launched due to serious database and hardware failures. World six is the first world to run for more than a year (about 2 years if I recall right)
June 2013: world 7 is announced and launched. It also runs for about two years while development on Salem continues.
Aug 2015: world 8 is launched as "Haven 2.0," internally known as "Hafen" with a major update to the client and server to integrate a lot of back end designs from Salem and several new game mechanics. World 8 ends with a "domination" event where players try to control an altar to end the world.
Feb 2016: world 9 is launched.
I'm sure I'm missing a few things with Salem development and such mostly because I don't recall approximate dates. Feel free to let me know and I'll update. Maybe jorb or loftar can provide some insight on missing information.