The Khan Academy Excercises section organizes math skills from ~grade 3 through to some grade 12 topics in hierarchy with dependencies. Each skill has a tutorial and quiz and students can earn stars as they demonstrate understanding.
This site expains the mathematics behind images, and occasonally supplements with applets, animations, or teaching materials. Still a little sparse, but what is there is well done.
A great site - essentially an interactive textbook for Grade 9 Alberta curriculum. Lots of java applets presented within written passages set up to lead through a progression of knowledge/skills. Resources for a number of other courses if you hit the [Home] link at the top of the page, but I have not looked through other areas of the site.
Practice questions covering most (all?) knowledge areas of the grade 9 curriculum. To track your progress you have to subscribe, but you can practice the questions themselves without cost.
An online tool that graphs multiple functions. Graphs can be linked to by URL, or downloaded as images (for use in tests/worksheets!). Can adjust window and zoom, but can not yet change graph/grid colours and scale is at the edges of the window rather than on the axes. Still in development, so may have features added in the future.