The Jet Propulsion Laboratory (JPL) is either located in Altadena, or Pasadena depending on who you ask in Los Angeles (It's really in Pasadena). It is managed by the California Institute of Technology and is NASA's lead center for robotic exploration of the cosmos. For programmers, one of JPL's more famous employees is Larry Wall who developed Perl there in 1987.
The 74-acre complex is open to the public, in a sense. Group tours are done weekly and every few times a month JPL has "Visitor Day" where people reserve a spot and go on 2.5 hour tours. In May (Or was it October?) they will usually have an Open House where they show off all their new stuff.
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, California 91109
(626) 354-4321