There are alot of different tools out in the market to score paper. Such tools as bone folders, line scorers used by scrapbookers are great for cardstocks and plain paper but does not work too well on 100% cotton paper such as lettra.
A good way to score a card is right on your letterpress machine. There are scorers and perforated lines you can buy that you lock up right in your chase. The only problem you might run into is denting your platen. You do not have to be concerned if you are not constantly scoring but if you are, you can always dedicate one press to scoring and perfing. But lets face it, for some people its hard enough to get your hands on 1 press. If you have a manual press such as a C&P pilot or Kelsey you can just use less pressure on the lever.
A recent post on the list in regards to scoring led me to these 2 alternatives.
Please feel free to share other ways of scoring paper.