I can't recommend the Tartans Authority Croft Weaver highly enough. I've drawn dozens if not hundreds of setts on it, and it's simply the best program I've used. There's one out for the iPhone, but it crashes a lot and the interface doesn't compare to the Croft Weaver.

I would recommend reading Mr Newsome's always-insightful article here, in his blog (written in his capacity as kiltmaker and tartan historian and scholar), which warns against using the Scotweb service.

And on a personal note, I can't speak highly enough of the value of putting in to the program various setts that already exist (specifically, tartans you like), then changing colors, adding lines, changing the number of threads, and so on until you've really got the feel of what tartan is in you; and all before designing one from scratch. For what that's worth.

Jake