JSTOR - Includes dozens of full-text journals dealing with math and statistics. Some date back to the 1800s, including the American Journal of Mathematics beginning in 1878. See Ms. Honey for access instructions.
National Library of Virtual Manipulatives - Covers everything from algebra blocks to integer chips to geoboards and includes unique tools to explore classic problems such as the Towers of Hanoi.
NIST Digital Library of Mathematical Functions - This is the companion site of the NIST Handbook of Mathematical Functions.