We have a class page maintained by the Auraria Library. It contains links to many good sources.
The following listing is due to Daniel Otero from his Math 300 page.
There are several sites which are particularly valuable to students in this course. Be warned that care must be exercised when using information you have obtained from the Web. Consider sources. Is the site based at a trustworthy location such as a university or government department? Are the documents written by scholars and experts, or by dilettantes and cranks? Frequently it is too easy to follow references given at a website than to use the documents without review.
For an illustration of the how the various branches of mathematics relate to one another, look at the Mathematical Atlas.
At the University of St. Andrews in Aberdeen, Scotland, is is the MacTutor History of Mathematics archive. This site contains biographies and images of most of the important mathmaticians throughout history as well as several other features. Generally, these have been drawn from the DSB, but some errors still persist. As with any other site on the Web, it should be used with caution. For example, look at the entry for Euclid. Who is the man in the engraving? Is it really Euclid? (Note: This is the picture on the initial page of this website)
For the study of Euclid, I recommend David Joyce's site at Clark University. He has a general history of mathematics and also a marvelous online version of the Elements.
An Archimedes page is maintained by Chris Rorres at Drexel University.
Len Berggren at Simon Fraser University has special emphasis on non-European mathematics. What is there is interesting, but this site has not been modified in quite a while.
At Trinity University, Dublin, you will find information on mathematicians of the seventeenth and eighteenth centuries taken from W.W. Rouse Ball's Short Account of the History of Mathematics.
Joseph MacDonnell of Fairfield University (another Jesuit institution) maintains a site on Jesuit Geometers.
If you are interested in the history of statistics, there is a site which has some original sources maintained by the department of mathematics at University of York in England.
For information on African Mathematics visit the African Mathematical Union.
There is much more. A starting point for exploration may be found at the British Society for the History of Mathematics at the University of Warwick, England.
The Canadian Society for the History and Philosophy of Mathematics has its own home page with some interesting information.
There is an extraordinary set of color photos of Greek works on mathematics and astronomy in the Vatican Library.
At St.-Andrews University in Scotland they have a large collection of files called the MacTutor History of Mathematics Archive. This site was originally rather unreliable but has improved a lot. Still, you should be careful to double-check information you find here or in any secondary source.
A special archive on Women Mathematicians is maintained at Agnes Scott College in the US.