Dean Murray, math professor at the Univerity of Waterloo (Ontario), has organized a website in conjuction with the Center for Education in Mathematics and Computing. The web site is Wired Math, which provides "fun resources and online games for teachers, parents, and students in grade 7, 8, and 9."
The FREE materials (on the order of supplemental worksheets) can be used either for enrichment or remediation. Though the topic areas are aligned with the grade 7, 8 and 9 Ontario Mathematics Curriculum documents, they easily are compatible with either the Washington State EALRs or the NCTM curriculum standards.
The site offers a series of games (from FUN BRAIN), though I would define them more as on-line visual experiences. Interwoven within the lessons, the "games" are accompanied by even more drills and follow-up exercises. For those teaching mathematics in grades 4-6, the site offers another series of "games" (called MathFROG), developed at CSMC.
Overall, the web-site's premise is that students will work towards improving both their mathematical skills and problem solving abilities through on-line games and paper-based drills. In addition to routine questions and skill-assessments, students are to be extended by the additional "challenging and enrichment problems." According to Murray, "There are questions for everyone. However many of the problems near the end are not routine and will require some degree of independence, judgment, originality and creativity to solve."
Though designed with a focus on grades 7-9, the Wired Math web site includes other links (somewhat self-referential...and unfortunately not MATHNEXUS) and a special link to CSMC's MathSwap, which is a web site for sharing classroom materials amongst math teachers in grades 9-12. The available materials are good, and include more worksheets (ugh!), discovery expereiences (yeah!), problem-solving experiences(yeah!), programs and files [GSP, TI, Fathom], and math project ideas (yeah!). Classroom teachers are asked to both use and contribute quality resources to the growing library.
Is the web site a good resource for you? It depends on your needs and pedagogical approach. Check it out and decide for yourself.