There are literally thousands of Internet (or World Wide Web) sites pertinent to mathematics. Most of the following are essentially "meta-sites," starting places for finding and linking to a wide variety of information online.
A database sponsored by the American Mathematical Society with links to different aspects of the association. The "Math on the Web" link provides a long list of Internet sites arranged by broad subject categories, such as "Guides," "People," and "Reference."
Provides links to a great variety of mathematical and mathematical-related web sites. Categorizes entries from General Mathematics WWW Servers in the US, in foreign countries as well as links to Pure Mathematics, Applied Mathematics and Mathematical Physics, etc., math gophers and newsgroups, all the way to Miscellaneous Web Pages in Mathematics. Maintained at the University of Wisconsin -- Marathon County.
Provides pages with links pertaining to a wide range of mathematics and mathematical-related areas including addresses, electronic journals, math department web pages, math education, math gophers and newsgroups, mathematical software index, specialized fields, and a LaTeX archive. Under general resources are links to various research institutions and professional societies, Internet sites pertaining to a broad spectrum of mathematical subjects -- both serious and fun -- such as a fractal movie archive, a mathematical tables web site, math magic, and even a page on math in the movies. Hosted and maintained by the Florida State University Department of Mathematics.
Platonic Realms is a collaborative effort of math graduate students and educators, and includes the Math Links Library with over 700 math links; the Quotes Collection (a collection of mathematical quotations arranged by author); the Platonic Realms Online Bookstore a collaboration with Amazon.com offering a extensive listing of mathematical titles; Mini-Texts, essays/monographs on general mathematical topics; and Pocket Topics, brief essays on specific and focused subjects like Zeno's Paradoxes, etc. Maintained by Math Academy Online/ Platonic Realms and edited by D. Sidney Smith at the University of Colorado -- Boulder.