http://olympiads.win.tue.nl/imo/books.html
A great list of problem-solving resources and classic books from various branches of mathematics, strongly recommended for all students with a passion for mathematics and especially interesting for those preparing for competitions. You may find some of the recommended books in our library.
http://www.artofproblemsolving.com
Very interesting for the students preparing for math competitions; check out the resources.
http://math.furman.edu/~mwoodard/mquot.html
"The Mathematical Quotations Server" from Furman University. Contains hundreds of more or less famous quotes about mathematics by mathematicians and not only.
http://www.ams.org/outreach/undergrad.html
American Mathematical Society's site for undergraduate mathematics majors. Contains general information about contests, summer programs, graduate schools and more.
http://www.ams.org/programs/students/undergrad/emp-reu
American Mathematical Society's guide to REU's (research experience for undergraduates - summer programs in mathematics). Unfortunately, among the many excellent programs from the list, very few accept applications from students that are not US citizens or permanent residents. However, the few that do accept applications from non US citizens or permanent residents are usually great opportunities for those interested in doing research in mathematics (and they are generally very well paid).
http://www-history.mcs.st-andrews.ac.uk/Indexes/HistoryTopics.html
Contains many links and articles about the history of mathematics
http://www-history.mcs.st-andrews.ac.uk/BiogIndex.html
Contains many biographies of mathematicians
http://gams.nist.gov/
A guide to available mathematical software
http://archives.math.utk.edu/undergraduates.html
Math Archives: contains a lot of interesting information and links for mathematics majors (e.g. summer programs, competitions, careers in mathematics, best graduate programs)
http://www.ams.org/mathscinet
The MathSciNet reviews: the most well-known source of mathematical reviews (books, articles, journals) on the web. A must for anyone considering a career in mathematics
http://genealogy.math.ndsu.nodak.edu
The Mathematics Genealogy Project has the high ambition of compiling information about all the mathematicians in the world. You can find here most of the people that have a doctor's degree in mathematics, the name of the university offering the degree, the title of their dissertation, their advisor, and can follow the "tree of advisors" until very famous names like Gauss or Euler.