I've used many different substrates, and I prefer Aspen. My snakes seem to like it the best as well.
I used these pellets for awhile, mostly because they were cheaper in bulk than aspen. It's absorbent and you can still do spot checks fairly easily. Once it gets wet it kind of flakes out.
This is what it looks like after some "use"
This was so long ago that I can't remember what they were made of. I DO still use pine pellets like this as cat litter. I cannot stand how standard cat litter smells and sticks to my cat's feet. lol, getting a little
I've also used Newspaper, but .. eh. It's not super absorbent, and if you use normal run of the mill newspaper, the print can rub off on your snakes.
*shrug* for whatever reason though, I always go back to shredded aspen. I don't like how expensive it is and how messy it can be, but it's not too terrible. ..Until I have a bin pulled out to clean while a snake is eating, leave it unattended for a second and come back to find the cat playing in it.