I provide player 'screens' in a lot of the RPGs I run. Since we don't play with a battlemat or miniatures, these lay flat on the table in front of the players instead of acting as a screen.
What goes on it depends on the game.
For D&D, it was 2 pages of how the various combat maneuvers work, what provokes an AoO, and a few important skill tables.
For CyberPunk, it is a list of firearms ranges, a full list of the skills in alphabetical order and what stat they are under (so when I call for a roll, you don't have to ask what stat it pairs with on the occasion that you lack said skill), the wound effects and stress effects charts.
For Vampire / WoD it is a single page with the names of the discipline powers at each level along with the die roll required for each discipline at that level, and a second page with some combat notes and how much blood is in what kind of critter and so on.
For GangBusters! it was a list of weapons, skills, and vehicles on a single page, along with a quick one-column description of the combat system.
For GammaWorld3e it was a copy of the ACT table on one page, and a list of different table results for different skill tests.
For Top Secret it was the combat modifiers, combat system, and the core sample guns on one page, and the martial arts tables compiled in a very small font on the second page.
For Rifts it was a cool piece of art and a scratch pad area for tracking MDC and damage and so on. No actual 'content'.
For Call of Cthulhu it was an overview of the Crit / Fumble rules, the San rules, and the healing rules.
