MIL's birthday is 12/19, we always get her a Red Lobster GC for her b-day because she and FIL don't go out to eat often (maybe 2 times per year), so it's an excuse to go to her favorite restaurant.
MIL is a computer person and is always printing pictures, so she can always use ink for her printer. I've also bought her a "membership" packet to shutterfly years ago so she could get a bunch of pics professionally printed.
FIL LOVES history books. This year we got him a book about haunted barracks, we've gotten him a book about a haunted mansion, books about the St. Louis Cardinals (old school Cards, not anything recent), war books (he was in Vietnam), etc. He also LOVES cooking, so we thought of getting him a smoker, but it was just not in our budget this year since DH was out of work.
My mom loves to be pampered so I could always get her a GC for a massage. She also loves technology (she has a laptop, iPhone, and will be getting a tab).
My s-dad is into hunting, 4-wheeling, and hiking. He just got a new Mustang so my mom suggested a Mustang shirt and/or "Mustang Parking Only" sign, and a book about some Rock music guy (don't even remember the book).