I have a friend who is in prison. I have compiled some resources of things I have have been sending to him throughout his time there.
Origami - There are tons of origami links out there you can print out and mail to an inmate.
They often have access to paper, but colored paper is harder to get access to.
We would often print a single-sided letter on colored paper so they had something to make things out of.
Solo RPG games (http://www.abandonia.com/en/gamebooks) Solo RPGs are booklets that are self-contained adventures. If anyone has read "Choose your own adventure" books, it is like that, except with some "Role Playing Game" aspects to them. Some of these booklets are long and require a fair bit of paper / ink. These are very popular, but you can only read through them a few times.
Net Epic: http://www.netepic.org/ is a gaming system for large-scale battles. While it normally uses playing pieces that you paint yourselves (which are not permitted in prison), you can print off paper 'chits' (paper playing pieces) that they can cut up and use. This has probably been the most successful of the things I have sent in. i have more information about that here:
Book titles and authors: Some prisons are part of an inter-library loan system. If you send in the authors and titles of books, they can often request the book. But they do not seem to have a good way of browsing the system, so without the author and book title, they cannot get books. Kind of sad, but that seems to be the way it is.
Inmates can usually own board-games, but often cannot have dice. Here is a link for something they can use instead of dice. It is a spinner that can be made from objects that they have access to.
My brother has created some games, both for educational purposes, and for fun. Here are a few of them
Tower Solitaire This is a solitaire game that helps with math skills. It is a fun way to generate math problems to solve.
Star Duel is a game of stellar warfare, conquering planets and destroying your enemies. It does use basic math skills.