Free and open source software
Free and Open Source Software (sometimes referred to as FOSS) describes a concept of declaring or licensing a creative work to encourage and ensure continued access, enhancement, and re-use by and for all. (See the Wikipedia article on free software.) This position has been popularly codified into the GNU Public License (or GPL) as well as the Creative Commons group of licenses. These licenses are sometimes referred to as "copyleft" in contrast to the more commercial term "copyright".
Copyleft sets itself apart from proprietary software by preserving fair attribution, but requiring the software to be open and any modifications to be offered freely, that is: to share-alike. Copyleft does not require (or even encourage) that contributors receive no compensation for their efforts (a common misconception because of the dual meaning of the word "free"), but that a work for distribution to the public remain free for others to continue to update, inspect, repair, share, and enhance. For this reason, one can see why free software is important for an open government.
Free and open source software is "viral," meaning that derivative works must be published under the same license. Note that to say that copyleft's "share-alike" clause reduces one's freedom is like saying that the U.S. Constitution reduces one's freedom. Of course, it is true in a way, but only in a very particular way: to ensure continued freedom for all.