I didn't think of most of these until I read through everyone's replies, but...
Grease
Ferris Bueller's Day Off
Sleepless in Seattle
You've Got Mail
Sound of Music
The Wedding Singer
and the one movie that I absolutely MUST stop to watch everytime I see it on tv...The Princess Bride! I own it and essentially have it memorized, but if I'm flipping through channels and see that it's on, I have to watch it!