(I haven't explored these in detail, but
they each had something to offer. Also try some of the activities by
following the "Math applets" links in the online demos list below
this list.)
TheMathPage (Private
educator-developed site with detailed explanations through calculus;
online practice with solutions)
Purplemath
(Private educator-developed site with detailed explanations up to
precalculus; no online quizzing)
Countdown (QuickTime videos on
many topics, from Loyola University)