I've been reading about Celtic Knots and finally learned how to create them. They're not as hard as they look, just time-consuming. Note that mine is not a wholly typical knot: Most are constructed from arcs (at least wherever the rope twists), while mine is much more angular.
This was created in about 3 hours with Adobe Illustrator 10.0. The AI file is included for download and can be opened with Illustrator or viewed as a PDF.
Information on the topology of Celtic knots can be found here