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ArXiv (pronounced "archive.org") is a repository for research papers in physics, computer science, mathematics and neuroscience. While many of the papers on the server are high quality, be warned that they are not peer-reviewed.
An online resource created by Wolfram Research, creators of Mathematica, it contains entries on numerous different concepts and terms related to mathematics.
An initiative of Drexel University, the Math Form provides a number of resources. A list of links specifically appropriate for undergraduates is located at: http://mathforum.org/kb/forumindex.jspa
Created by the University of St. Andrews, this site contains a number of helpful resources including biographical information on mathematicians, timelines, and quotations.