It was a piece of illustration board with a too-heavy coat of gesso that was headed for the trash can when I decided it could still be useful.
The board had warped and the gesso had cracked because I got carried away troweling it on in an effort to create texture.
I had done some pieces previously where I wiped paint over paper that had been coated with acrylic medium for a sort of aged/distressed look, so I decided to try that here.
I glued a regular piece of printer paper on top of the dried gesso and coated it with gel medium, then when it was dry, I used a baby wipe to add a light coat of acrylic craft paint to bring out the texture (I use the cheap stuff - $.33 for a two oz. bottle on sale at Michael's). I like to use a medium gray paint for this type of thing because it makes it very easy to add color in Photoshop.
Once it dried, I scanned it and added adjustment layers (levels and hue/saturation). Now whenever I want to add it to the background of a collage, I just tweak the sliders in my adjustment layers, sometimes adding a layer of color over the top, and almost instantly I've given my collage a handmade look without having to create a brand new background from scratch every time.
A couple of good resources I have in my own library for creating handmade elements that can be incorporated into mixed media pieces (digital or otherwise) are Collage Lab and Print & Stamp Lab. I also like Art At The Speed Of Life for tips on creating somewhat more complex collage backgrounds in batches instead of one at a time so you always have a jumping off point instead of a blank canvas staring at you when you're ready to create.