I am myself using R (www.r-project.org
). I has excellent publication-quality graphics and a huge number of statistical tools. However, it is in the first place a matrix-oriented programming language, so you have to learn a bit in order to feel yourself convenient with it. It is definitely worthwhile if you inted to do a lot of statistcs-related stuff. There is also excellent documentation (the best I have seen for OSS) and an active mailing list.
You may also try gnuplot. It is a bit old-fashioned but a robust plotting program. Does not do much statistics.