MathisFun has great walk-throughs of concepts for Algebra. There are practice problems in the “Your Turn” section at the bottom of each page.
CoolMath has a lot of example problems with explanations and a couple of practice problems for each topic.
SOS Math has a couple of examples for each of their topics as well as a few practice problems with answers.
Wolfram MathWorld covers topics from Algebra through college-level mathematics. The information is pretty high-level and is probably best for older students.
SparkNotes Math has notes about all levels of math, divided up by topic, complete with practice problems and solutions.
Purple Math has a goal of explaining math concepts in a user-friendly way that doesn’t sound like a textbook.