1) Food and drink. Something that's neat to eat, not too horribly unhealthy, and easy to share. Grapes and bite-sized pretzels are ideal.
2) If somebody else is driving, a good book.
3) Or knitting.
4) Or crocheting. (My husband does most of the driving.)
5) Books on tape if it's just grown-ups in the car.
6) Read aloud from a "Miss Manners" book - it always makes us laugh, and sometimes triggers a good discussion.
7) XM radio - often it's the only way to follow the Red Sox when we're traveling.
8) Bring some math problems to work on with my husband.
9) Drive at night. You will see the countryside from a different perspective.
10) Have a DVD player for the kids. Seriously, it's the only way to stay sane with small children on a long trip.