Ross Tech very kindly do a VCDS Lite freebie version, so you can just buy a cheap lead and use it. In fact, most ebay auctions link to the Ross Tech website!
There are hundreds of free packages out there (including source code it you're that way inclined), but most don't work with the VAGCOM leads, and the others just aren't reliable enough to use.
I don't know what Ross Tech did in their software, but when you compare it to other stuff on the market it shows itself as being the real dogs danglies.
