First, the source code for Platitude now supports the Mac. No real installer yet, but at least it runs.
Second, an email correspondent (Dethe Elza) sent along a few Celtic knot pages:
- Celtic Doodler, a Java applet which I've never seen before, but which is very similar to Platitude in approach.
- How to create a celtic knot, an explanation of a paper-and-pencil technique for making these knots.
- A Truetype font for creating Celtic knots, which was mentioned in the comments, but deserves a link here.