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.
MathisFun has links to many different topics broken down by grade level. There is also a link on this page about Common Core. This site does contain basic math topics.
The last four chapters of this open source precalculus book are a great trig resource.