I think that showing gratitude is one of the kindest, nicest responses that someone can give, and something that I don't do as much as I should. The best way to do it, I think, is with a handwritten note: I don't think my family bought a single card (birthday, holiday, or thank you) when I was growing up because my mother had a card making production factory going on in our home (the single employee: me). And I really miss doing that now!!
Some cards that I think are perfect for that "end of semester" thanking:
 Homestead Thank You notes from Paper Source
 Frances Thank You card from Rifle Paper Co.
 Banner Thank You notes from Paper Source
 Thank You rubber stamp from Scout's Honor Co.
Side story: I saw the Paper Source cards yesterday when Dan and I went out on an Adventurous Date in San Francisco and spent half an hour in Paper Source. Aren't they adorable?? The one with the little flags is like yelling out GRATITUDE and I find it hilarious.
If you want to go paperless, I think that Paperless Post has a really great selection of ecards and evites, all of which are free!! My favourites are the Snapshot card (photo and wording editable, and I've already sent 2 of those out to my family), Sweden in the Summer, and Smart Cookie (perfect for graduation!!)
I WILL make an effort to write all my supervisors and professors a thank you card this semester because they really have been overwhelmingly accommodating this semester. Maybe even make them by hand? Art project for the weekend :) Happy Tuesday-- it's been a long day for me but these pretty paper products are getting me excited again!