Wolfram MathWorld covers topics from Algebra through college-level mathematics.
SparkNotes Math has notes about all levels of math, divided up by topic, complete with practice problems and solutions.
MathisFun has Calculus materials, they're just a little hard to find from their main page. There are practice problems in the “Your Turn” section at the bottom of each page.
SOS Math has a couple of examples for each of their topics as well as a few practice problems with answers.