Sci.math newsgroup is an international "bulletin board" for mathematical discussion. It usually seems to be at about a graduate student level. You will find a list of postings you can read by clicking on them, and you can make postings of your own. If it doesn't work, find "Mail and News preferences" and enter your news server. (At Illinois State University, it's news.ilstu.edu.)
Yahoo's list of math and science education sites. (Yahoo is one of several Web search facilities; for others, press "Net search" above.)
Dr. Math. Students answer mathematical questions, from college level down. Read questions submitted and their answers, or submit your own.
Mathematics Archives at the University of Tennessee, Knoxville. Software for mathematics and math teaching at college and graduate level, with links to other sites and K-12 materials. This is where "Group Analyzer" for 407 and 336 came from.
WWW Virtual Library (Mathematics) Including Specialized Fields, Departmental Web Servers, General Resources, Mathematical Software Index, Mathematics Gophers, Mathematics Newsgroups, Preprints, Electronic Journals, High School Servers, Addresses, Bibliographies, TeX Archives, and Math Education.