Space agencies were some of the first places where you could find open source software "in the wild". Being natural early adopters, cash-strapped and very inquisitive, the agencies naturally took to the open source concept. Additionally they were some of the first users of ARPAnet and subsequently the Internet, which which lit a fire under the open source movement. Today this marriage between rocket science and open source saw its latest victory with the landing of NASA's Phoenix Mars Lander.
The amount of software coordination and lack of commercial software geared to the aerospace industry continues to make custom software necessary for almost every mission. Linux, the gcc compiler, visualization libraries and more have allowed NASA to expand the capabilities of their missions beyond their greatest expectations. Congratulations to the team at JPL and the University of Arizona for their phenomenal accomplishment.