Now, you might want to check out The Writer's Store to score some classic giftables for the screenwriter in your life. (I'm partial to the b/w "Writer" ballcap, myself.)
If they haven't read them already, any of these classic books would make an admirable addition to your screenwriter's bookshelf:
Save the Cat! by Blake Snyder
Screenplay by Syd Field
The Hero's Journey by Joseph Conrad
Or a subscription to Script magazine might fit the bill, too.
You could also get them software (like FinalDraft), coverage, a writing class, or a trip to a conference where they can continue to pursue their dreams.
Paper, highlighters, 10" x 13" envelopes, print cartridges, 1 1/2" brads and an audio recorder would all be helpful, too.
The best gift, however, is reading their scripts. Honestly, there is nothing more valuable than when people take the time to read my work and give me feedback. Any success that my work has is as a result of the comments, positive, negative or off the wall, that others give me.
It really is a collaboration. A gift, if you will.
What's gift you'd most like to get? What makes it so special to you?