Yesterday I went on a hunt to find some Celtic knot patterns to add to filet crochet. I started out looking for cross stitch patterns since they can be translated to filet, but I didn't really find the pattern I was looking for.
I did come across a very cool program (http://www.clanbadge.com/knots.htm) for "Celtic fonts," which doesn't involve the alphabet at all. Instead, it overlays snippets of Celtic knots over your keyboard so that you can make an endless number of Celtic knot designs. If you make lots of Celtic knot designs, that program would probably be worth the money. However, I was looking for a cheapo version.
Then I remembered a nifty design tool I had found several years ago; as you draw with your mouse, it makes mirror images of your drawing all over the place! It's located here: http://www.nga.gov/kids/zone/, and the two I like best are Wallover and PaintBox.
Yes, these are for kids, but if you're playing around with symmetrical designs, they could make your life much easier. Note that for most of these, you can only print them, but I imagine you could print them on graph paper (and you can print your own graph paper here: http://www.printfreegraphpaper.com/)
You'll need the latest version of Adobe Shockwave Player (a free download) to check it out. Even if you don't use it to design, it's still lots of fun to play with!