Students often need to research biographies of mathematicians, and complete, authoritative biographies are somewhat difficult to find on the Internet, especially in one web site. In my opinion, the best site with full information about more than 1500 mathematicians is a very simple one from the MacTutor History of Mathematics Archive, created by John J. O'Connor and Edmund F. Robertson from the School of Mathematics and Statistics at the University of St. Andrews in Scotland.
This very straightforward web site includes an alphabetical index as well as chronological indexes, so that students can also search by time period. For each biography, there is at least one picture of the mathematician, information about when and where he/she was born and died, a full printable biography, links to other web sites for more information about the person, a list of awards, some quotations by the famous person, links to obituaries from various newspapers, and a list of the references used to write the biography.
There are a number of other interesting links included in this excellent math biography web site. My students often had to research mathematics history and this site includes a link to "History Topics," which includes information about Ancient Babylonian, Ancient Egyptian, Ancient Greek, Arabic, Chinese, Indian, Mayan, American mathematics, and Math in Scotland (the owners are in Scotland). I wish I had found this web site many years ago!
Other interesting links include Mathematicians of the Day; Birthplace Maps, showing where in the world mathematicians were born; a listing of Mathematics Societies and Honors and the winners' names; a lengthy Chronology of Mathematicians by Decade; a Timeline for mathematicians; and finally a search form for the entire web site.
This math biography web site from MacTutor History of Mathematics and the University of St. Andrews is a wonderful hidden jewel for math students and teachers alike.
Searchable (site searchable)
Recommended for Students in Grades 6-12