Everybody knows about the paper towel roll hides. I usually flatten them a bit, crimp the ends, and stuff bits of crumpled up newspaper in them for extra security.
Old, unused dishes? Snakes can make use of them. These plates were a bit tricky to break. The plate on the right was carefully chipped away bit by bit, the plate on the left broke away clean just the way it is with the first tap of the hammer. I ruined a few of them when they split cleanly in two, or shattered into many small pieces. But when they break right they make great warm hides.
The lids of plastic containers make great hides. Some snakes prefer to get down into a hide with a hole drilled through the top, some prefer the hides with the side cut out. Some don't want an entrance at all, just set the lid down upside-down and the snake will coil up in it. My Rat Snakes in particular seem to favor the upside-down lid. Especially with a bit of cover overhead, such as artificial plants.
Sections of old PVC pipe make for great hides as well. Again, stuffing them with bits of crumpled up newspaper can make them more appealing to some snakes.
You can split the PVC pipe down the middle for hides that wont roll around.
Or even cut them at 45 degree angles and glue them back together to make corner hides.
If you want to get a bit more creative you can start combining parts. These slightly elevated hides have proven to be very popular with my juvenile Water Snakes and Rat Snakes. The one on the right was designed specifically to take up a lot of space in a 28 quart tub, while creating usable vertical space and multiple zones within the relatively small tub. An added benefit to building these types of hides is the additional weight and stability. The simple plastic lid hides are great, but very easy for snakes to move. With features like these you are much more likely to find things more or less the way you left them.