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/

Open Problems in Discrete Mathematics  
   From the SIAM DM activity group newsletter and other sources.
   http://www.siam.org/siags/siagdm/siagdmopen.htm

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  
   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.
   http://www.math.binghamton.edu/zaslav/Matroids/matroidprobs.html

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

Combinatorics Problems  
   Combinatorics problems selected from math olympiads.
   http://www.geocities.com/CollegePark/Lounge/5284/math/combinatorics.html

Problems in Graph Theory and Combinatorics  
   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.
   http://www.math.uiuc.edu/~west/openp/

Problem Collection in Combinatorial Optimization  
   Maintained by the EGRES group. HTML/LaTeX/text format.
   http://www.cs.elte.hu/egres/design/generated/problems.html

Josh Cooper's Math Pages : Open Combinatorics Problems  
   Offers descriptions of discrepancy, Euclidean Ramsey Theory, geometric graphs and hypergraphs.
   http://math.nyu.edu/%7ecooper/combprob.html