There are SEVERAL. I've used these 2 in designing tartans for several purposes, inc. a kilt organization.
One popular one:
http://www.weddslist.com/cgi-bin/tar...urce=rb#result

My favorite (though the WHITE looks GRAY):
http://www.houseoftartan.co.uk/inter...gner/index.htm
This one is VERY easy to use and adapt/modify it.

On BOTH of these, you are best screen capturing the image/design or printing it, as neither really lets you SAVE them.