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Great resource, thanks. Another great resource is, of course, the Khan Academy. See their precalc videos here:

http://www.khanacademy.org/#Precalculus

And their calc videos here:

http://www.khanacademy.org/#Calculus

I just wrote a short blog post on different kinds of places to find math online: http://inperc.com/blog2/2011/02/24/math-online/.

I noticed you included planetmath in your list of encyclopedias. They used to be an excellent source for definitions and theorems in pure math. But for the past 3 years, their search has been broken and their rendering has been slow (each page view lands on a yucky page of unrendered typesetting syntax, which is replaced by a blank page for more than a few seconds before some 'jsmath' plugin does something?). They hemorrhaged most of their users during that period. I found some notes from their board meetings, and they've know about these issues, they discuss them continually, but they've done nothing for 3 years. It's really sad, because it could have been an alternative to wikipedia for pure mathematicians.

Another hidden gem for academic mathematics is york university's ask a topologist: http://at.yorku.ca/cgi-bin/bbqa . Regulars on this forum routinely answer and discuss graduate and research level questions (in algebra and geometry too, not just topology).

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