These + a paintbrush for glue are the constant tools I always use for collage. I've become quite attached to them. I think we all settle to using a cherished number of tools from the beginning, where we can want to use ALL THE THINGS. You know, like stamps and punches, and all that stuff.
Green templates: I worked very briefly as a receptionist right out of high school, at an interior design firm. One of the employees had, weirdly enough, been involved in sports at my high school-- at least I think it was sports. Either way, us meeting was unexpected. He gave me several of these green plastic templates; circles, ellipses, square things, etc. I use these two all the time.
Utility knife and X-acto knife: Utility knives are great when I want to cut book board or anything that's got some thickness to it. I'm forever losing track of them, though. An X-acto knife's sharpness is crucial to my work; I use it pretty much on every piece. I have a refill pack of 50, so when the blade starts to dull a bit I can easily replace it with a new one. I have a lot of knives for some reason, but this one is my favorite because of the grip.
Bone folder: This particular one is near and dear to me; it's another tool I've used for years, and I started using this one in college. It has history to it. And nothing is better than a bone folder when you need to make a sharp crease in paper, or to smooth out tracing paper layers on a collage.
Bookbinder's archival PVA: My adhesive of choice. I've bought ModPodge, which I know is popular in the collage world, but never really got around to using it more than once or twice. And I can't even remember it clearly. PVA's just the perfect adhesive, and using the archival one means a little bit more longevity for my collages. I used a lot of acrylic matte medium back in the day, but as I no longer paint it on the surface of the paper to adhere things, I don't use it.
Ruler: Ahhhh. I love rulers. They appeal so well to the precise, OCD part of me, and is one of those other tools that is crucial for cutting paper down to size, trimming paper, and straight lines. My large metal rulers were too unwieldy, but I found this one, that's actually pretty short-- the edges end just after the photo, at Blick.
Then, of course, there's what surfaces I like to work on...but I'll save that for another time. Do you have any favorite or essential tools?
BTW, if you're looking for a more affordable way to purchase some of my work, I've got a shop over at Society6 , where you can buy prints of many of my works. I'm adding new ones all the time. You can also buy cards, iPhone cases and skins, travel mugs, cards, notebooks....and even a bath mat!