I found these plastic binder envelopes at Costco this fall and I love them. They can hold up to 100 pieces of paper and are strong and durable. I have to admit I don't actually use them in binders as I find they tend to stick out too far when the binders are closed (just a hint of OCD when it comes to my classroom) but if I cut off the 3-holes with scissors they fit perfectly in the white magazine holders from IKEA.
The best part about these babies is the price. It cost $10.00 for 25 plastic sleeves and I can think of so many uses for them. At this point I am using them to hold all the little bits and pieces I use for each of my Math units. Some of the envelopes hold the pieces for our interactive notebooks while others hold the cards needed for Math games. I was actually surprised at how much they held. The nice thing I can easily label them with a Sharpie and since the plastic is nice and smooth I can take the Sharpie off using a whiteboard marker later. (If you haven't tried this yet by the way you need to. You can remove Sharpie by colouring over it with a dry erase marker and then wiping it off.
I had hoped to use them as a write on/wipe off cover for worksheets but I think they are actually a little too thick (as in hold too many papers) to keep one piece of paper in one place for students to work on. What would you use these for? I would love some more ideas.