The MacTutor History of Mathematics site is an award-winning collection of biographies of mathematicians and articles about mathematics history.

Wikipedia has a Mathematics Portal full of biographical information. It is also easy to search for a particular name.

The Mathematics Genealogy Project, hosted by the North Dakota State University, catalogs the “family tree” of mathematics, tracing mathematics Ph.D. holders back through their dissertation advisors to Gauss, Euler, Hilbert, and other great mathematicians. You can even search for your own professors.