MathisFun has great walk-throughs of concepts for Geometry. There are practice problems in the “Your Turn” section at the bottom of each page.
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. They cover elementary school topics, Pre-Algebra, Algebra, and Geometry.