Sites

Open Problems for Undergraduates (Popularity: )
http://dimacs.rutgers.edu/~hochberg/undopen/
A collection of open problems in Discrete Mathematics which are currently being researched by members of the DIMACS community.

Open Problems in Combinatorics (Popularity: )
http://www.combinatorics.net/problems/
Features links to newsletters, workshops, and papers.

Problems in Graph Theory and Combinatorics (Popularity: )
http://www.math.uiuc.edu/~west/openp/
A collection maintained by Douglas B. West, of the University of Illinois. Features brief introductions, along with references cited. Topics include matching and independence, connectivity, graph homomorphism and circular coloring.

Past Open Problems (Popularity: )
http://www.math.uiuc.edu/~west/pcol/pcolink.html
From the SIAM Activity Group Newsletter in Discrete Mathematics. In PostScript. Compiled by Douglas B. West.

Some Problems in Matroid Theory (Popularity: )
http://www.math.binghamton.edu/zaslav/Matroids/matroidprobs.html
Thomas Zaslavsky of Binghamton State University of New York presents brief descriptions and references. Find information on Bonin's Projective Bound including the maximum r-flat, covering and packing by flats.