A book of mathematical puzzles by Peter Winkler.
Graduate course videos in open, royalty free WebM format.
An introduction to combinatorial mathematics by Kenneth P. Bogart.
Before his death in 2005, Ken Bogart was in the final stages of completing an NSF-sponsored project about the teaching of combinatorics through Guided Discovery. The textbook which grew out of this project is available via the link above.site.
The Center for Mathematics and Quantitative Education supports the teaching and learning of quantitative subjects across a variety of contexts. Collaborating with national and international projects, the Center provides resources that address a range of pedagogical concerns.
This is perhaps one of the oldest, and most complete educational resources on the web. It is supported by NSF and based at Dartmouth. Laurie Snell is the main person for its existence and continued excellence. It provides up-to-date resources of chance-related topics, including the Chance News, a searchable Chance database, and various versions of the Chance course taught at different colleges and universities. It's a perfect resource for an introductory statistics class, or something similar.
This is a five-year project based at Dartmouth, funded by the NSF which involves almost all academic departments at Dartmouth College. The main purpose of the project is to develop modules for interdisciplinary courses where mathematics is a tool. There are already a number of courses offered under this project.
This is an implementation of the University of Rochester's WeBWorK, which allows each student to get their own unique set of homework problems and receive immediate feedback on the correctness of answers via a web interface. The above link is to the courses where WeBWorK has been implemented.