Last year (Spring 2005), one of my teachers has asked me to research software for middle-school mathematics (I have focused specifically on pre-algebra). Unfortunately, after several hours of focused surfing, I can only report that there simply is not much out there. These are the limited results I have been able to compile.
These are mainly full fledged portals and it is unclear if they offer specific middle-school orientated interactive tutorials: MathPro5 MyMathLab
This software is probably too much for middle-school, but since I've run across it, I might as well lay down the links so they don't get lost... Incidently most of this stuff is OpenSource software that runs on Linux, unfortunately this is probably another hurdle toward making these useful in a middle-school classroom.
This may actually be borderline useful in an 8th grade classroom: GraphCalc for Windows & LinuxThese links are now broken (this is in fact a common phenomena of the web, usually termed "link rot"), I may have included a google search link because they may eventually come back to life somewhere else on the web.