SIGSAM
ACM Special Interest Group in Symbolic and Algebraic Manipulation. Site has details of meetings, reports and other links. http://www.acm.org/sigsam/
SymbolicNet
Information centre for symbolic mathematical computation. Links to research and software. http://www.symbolicnet.org/
Groebner Bases
Definitions and examples, with Java applet. http://www.geocities.com/CapeCanaveral/Hall/3131/g_bases.html
Computer Algebra Nederland (CAN)
Coordinates the use of computer algebra in education and research. Research reports, newsletters and historical material. http://www.can.nl/
Communications in Computer Algebra
The ACM SIGSAM Bulletin. Some tables of contents and sample articles. http://www.apmaths.uwo.ca/~rcorless/sigsam/
The Computer Science Directory currently has categories, which include links along with indexed pages.