Open research-level problems in Geometry. Recreational problems and puzzles have their own categories.
Sites 4
- Open Problems Collected by Jeff Erickson. Mainly in geometry.
- Open Problems from the Geometry Junkyard Compiled by David Eppstein of the University of California at Irvine.
- The Open Problems Project A project to record open problems of interest to researchers in computational geometry and related fields.
- Steiner Trees Open Problems with Steiner Trees, maintained by Joe Ganley.