At least two come to mind:
1. The phone number of a licensed and insured tree care professional
2. The phone number for your insurance agent... just in case.
Having said this, it very much depends on what you mean by "large" and whether or not it's one tree or ten, and of course location. If we are talking a single 3-6" diameter and it's not going to hit anything (or anyone) coming down, the potential for bad things happening is relatively small -- though do bear in mind a tree that size can still kill very easily. If it's a single 32" half-rotten pine 5' from your house, walk away. It's not worth the risk.
Also think about the cost in time and equipment for just one tree. It is not trivial to learn how to cut down trees safely nor is it a quick and easy job! For larger trees you need a chainsaw big enough to cut safely, and that can easily set you back US$6-800. Add safety equipment and all that and you are looking at $1k easily. And probably 3-5 hours work for a single large tree when all is said and done.
In my own case there were so many trees that needed to come down (the bark beetle in this part of the world has killed 80% of pines) that it was very much worth it to purchase the equipment and become proficient at felling trees. I started very small in areas where there was no risk to property or people and worked my way up. I can now comfortably take down 32" diameter trees and it's rare they don't go exactly where I want them to. A really good book on this subject, in fact one I would consider required reading, is "To Fell a Tree" by Jeff Jepson.
Finally: Many people seem to think that cutting a tree down is not a big deal. The physics involved are surprising complicated, the forces are huge, and the number of variables that can really ruin your day are enormous! There's a reason it's one of the most dangerous occupations around and there's a reason so many non-professionals are killed or seriously injured attempting to cut down trees.