Quoted from the recently release AMS opinion

"Mathematical software has greatly contributed to mathematical research, enabling exciting advances in mathematics and providing extensive data for conjectures. Perhaps three of the most well-known applications of computation to mathematical research are the resolution of the four-color conjecture by Appel and Haken in 1976 (though it is now reproven with less need for computer verification by N. Robertson, D. P. Sanders, P. D. Seymour and R. Thomas), Thomas Hales’s proof of Kepler’s conjecture, and the formulation of the Birch and Swinnerton-Dyer conjecture, which grew out of extensive numerical computation.

Open source software, such as TEX, Mozilla Firefox, and Linux has had a profound effect on computing during the last decade, and we hope that open source mathematical software will have a similar positive impact on mathematics."

Read the full Opinion at:
http://www.ams.org/notices/200710/tx071001279p.pdf