What are some valuable Statistical Analysis open source projects available right now?

Edit: as pointed out by Sharpie, valuable could mean helping you get things done faster or more cheaply.

The R-project

http://www.r-project.org/

R is valuable and significant because it was the first widely-accepted Open-Source alternative to big-box packages.  It's mature, well supported, and a standard within many scientific communities.


Some reasons why it is useful and valuable 
There are some nice tutorials here.

