absolutely- when I applied to grad school I sent all of my letter writers a really nice red wine, carefully selected from my favorite wine shop. maybe that's how you know they're the perfect letter writer- when you've shared enough wine with them to know their preferences.
seriously though, a nice bottle of wine is always useful, and if they don't drink they can bring it to/serve it at a party
get them something that can be consumed so they don't look at it and go "no where the heck do I put this?"
starbucks gift cards are also popular - or a gift certificate to a favorite lunch place