Diamond originally posted the thread as a place to put maps for published AH works, and hence put it in the Books and Media forum. However, since then it had become a place to post any random maps you feel like, and generally show off your cartographic skills. This gradually led to the move of maps based on published works to new, specialised map threads.
Since the times of the original map thread, there have been eleven successive map threads for posting and showcasing various alternate history maps.
Glen at one point tried to create Map Threads for every forum (pre-1900, post-1900, ASB, etc.) but these were not as successful as the single unified Map Thread approach. A Map Series Thread, ASB Map Thread, and Future Map Thread do exist (in their forums respectively) however they are mainly used for crossposting ASB, Series, or Future maps from the huge, all-encompassing Map Thread.
a.) The main threads
These can all be found in the Maps and Graphics forum.
Map Thread II - the second in a long line of venerable Map Threads. Posted by Ian on October 24, 2006, this Map Thread was known for Imajin's Legendary Monitor and debates on the topic of UCS and Diamond's World Basemap. It was closed on September 15th, 2007, to preserve server resources.
Map Thread III - the third continuation of the Map Thread, posted on September 15th, 2007 and closed on May 9th, 2008.
Map Thread IV - the fourth Map Thread. It was created on May 9th, 2008 with the death of Map Thread III and closed on November 16th, 2008 to make room for Map Thread V.
Map Thread V - the fifth Map Thread was posted on November 16th, 2008 and closed on September 25th, 2009. It was the first Map Thread to venture to 500 pages before closing.
Map Thread VI - created on September 25th, 2009, the sixth Map Thread actually made it to nearly 550 pages before closing on October 24th, 2010.
Map Thread VII - the seventh Map Thread, created on October 24th, 2010, continued the tradition of 500-page Map Threads. It was closed on June 23rd, 2011.
Map Thread VIII - the eight reincarnation of the Map Thread was remarkable for being started by Burton K Wheeler rather than Ian. It was started on June 23rd, 2011, and closed with the (controversial) start of Map Thread IX on December 21st, 2011.
b.) The “spinoffs”
Regular posters in the Map Thread are informally known as the Mappist Cabal.