This was one of those flash obsessions that won't let go until they've forced you to give them form. I don't like keeping them inside (I always need the room for the next one that'll inevitably come along). I've always liked Celtic knotwork anyway.
The font was made with METAFONT. The algebraic relations and equation solving features of Knuth's program really shine in knotwork drawing.
Here is a really nice single-strand knot. Proper celtic knots can be very intricate, but are always made up of a single strand.
A couple of crappy knots I made during testing. These are not single strands. Like I said ― crappy. The font helps a lot, but it won't do everything for you. The top one is a nice, two-strand knot, but overly simplistic. The bottom one is much nicer and less symmetrical, but has too many strands (a nice long one and three short rings).