Sail design is no black art, but some sailmakers certainly want to keep a veil of mystique to it. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />
I would not call it good, but there is sailcut CAD.
http://www.sailcut.com/Main_Page While I dont think it is good, it will do the job decently once you learn it's quirks. If you get serious with it, we can discuss things to watch out for before you finalize the design.
Other software packages, which professionals use, costs money. Prosail is one package, SmSw6 is another. Both have a learning curve which is probably steeper than sailcut CAD.
What sail are you planning to have a look at? I have certainly always tought the hours we put into sailmaking was worthwhile both for the learning and for the bank account. As I have said earlier here, you develop a very different eye for sailtrim after you have designed, built and recut a sail a few times. We wrote a small article on one of our sailmaking projects some years ago:
http://www.thebeachcats.com/Article213.htmlThat sail was based on a design Marcus Towell shared but which we adapted for our mast and sailing conditions.
