Open Problems for Undergraduates A collection of open problems in Discrete Mathematics which are currently being researched by members of the DIMACS community. http://dimacs.rutgers.edu/~hochberg/undopen/
Past Open Problems From the SIAM Activity Group Newsletter in Discrete Mathematics. In PostScript. Compiled by Douglas B. West. http://www.math.uiuc.edu/~west/pcol/pcolink.html
Some Problems in Matroid Theory Compiled by Thomas Zaslavsky. http://www.math.binghamton.edu/zaslav/Matroids/matroidprobs.html
Problems in Graph Theory and Combinatorics Collected and maintained by Douglas B. West. Open problems are listed along with what is known about them. http://www.math.uiuc.edu/~west/openp/