NOTE: Yacas and yagy development has moved to github. The new repositories are located at https://github.com/grzegorzmazur/yacas and https://github.com/grzegorzmazur/yagy . The mailing list has been moved over to https://groups.google.com/forum/#!forum/yacas . The downloads section at sourceforge will be kept up to date for the foreseable future.
Yacas is an easy to use, general purpose computer algebra system. It uses its own programming language designed for symbolic as well as arbitrary-precision numerical computations. The system comes with a library of scripts and extensive documentation.
Yagy (http://sourceforge.net/p/yagy) provides full yacas experience wrapped in a simple and intuitive graphical interface.
Keep it up!