Why this place on stack exchange is called "area51"? Is it a special idiom in English for some places where things are being developed? Does 51 have some special meaning besides being just a number?
Area 51 is a military base in Southern Nevada often found at the center of UFO stories.
Area 51 is a military base, and a remote detachment of Edwards Air Force Base. ... The intense secrecy surrounding the base, the very existence of which the U.S. government barely acknowledges, has made it the frequent subject of conspiracy theories and a central component to unidentified flying object (UFO) folklore.
As to why StackExchange called that site Area 51?
From the Area51 FAQ, "We like the little aliens."
See http://en.wikipedia.org/wiki/Area_51 for information about Area 51. In a nutshell, it's a humorous reference to a place where strange, secret things happen.