One of the joys of going to see a Pixar film at the cinema, is that you get to watch a short first, and generally you forget until you’re on the way and you think, oo, I wonder what it’ll be this time! You have a vague idea of the film you’re going to see, but this is a totally unknown treat to warm you up.
Over the years there have been some classics, but I do have favourites. I found some of them on youtube, for others I’ve just had to put the trailer, but you can find them on the Pixar Short DVDs they sell, or I think on the DVDs of the films they were in cinema with. (A full list of Pixar shorts and their relative feature length films can be found here)
I went to see Finding Dory last week, and really enjoyed that short, though it’s not listed below because in a way it feels too soon to decide if it’s a classic!
Anyway, enjoy.
Lava (before Inside Out)
Day & Night (before Toy Story 3)
The Blue Umbrella(before Monsters University)
For the Birds (before Monsters Inc)
Luxo Jr (before Toy Story 2 (but made in 1986))
ineke.co.uk 
Anything to add...?