Many adventures use player handouts: things the GM prints and hands to the players to look at. I'm personally a big fan of these, and an adventure can't have too many of them. They immerse the players in the adventure in a tactile way. But what materials make for a good player handout? What good are they? There are many answers!

* Art. First and foremost, player handouts are designed to be looked at. Although it's possible (and, in fact, common) to have a text-only player handout, it should still be pretty to look at. Generally, player handouts should be grouped with your art pieces (and handled by artists, if possible) rather than ordinary text layout. Consider a background and font that makes it look like the piece would look in-game. Rough it up a little, if it's a balled-up or discarded handout. 

* Roadmap. A common player handout is a player-friendly map that shows the immediate area. This allows the players to note sites where they might find clues, or locate other adventure hooks. Ultimately, this provides a roadmap to adventure. Several blogs back, I noted that you can have a sandbox-style adventure that provides some subtle direction to the players about which order to hit the adventure locations; the roadmap helps emphasize this. If the players see that the town of Farholllow is quite a bit past the Ogre Caverns, they're more likely to stop at the Ogre Caverns as it's "on the way."

* Rules Reference. If you've got an infrequently-used subsystem (like a vehicle chase, starship combat, or verbal duel), then a handout setting forth the rules is helpful to players. This can be the most text-based of all, and encouraging the GM to print out a few copies is wise. My latest example is my A Bad Day for Trolls adventure, where I include a handout explaining regeneration for the players (as they're all trolls, it's an important rule to understand).

* Checklist. A very useful role for a player handout is a checklist list for the players. If they have to gather information about 4 NPCs, a list of the NPCs is helpful (particularly when, as is common for RPGs, the spelling or pronunciation of the NPCs isn't intuitive). Components of a ritual, clues to a murder, and so on are all good items for a reminder-type player handout.

* Puzzle or Trap. Sometimes the handout presents a puzzle to be solved, conveying exactly what the characters see in hieroglyphics, a wall tile, or mechanical dial. Having the puzzle right on hand to reference is super helpful to solving it. Sometimes, this type of handout can be a trap of its own: if you provide pieces of paper to be rotated, the GM might watch to see whether they're rotated into the wrong configuration and have a trap trigger. The most classic type of trapped handout is one with the words "explosive runes" embedded in the text; the GM can always tell, even by facial expression, when a reader gets to that point. That's the time to call for Reflex saving throws!

* Reminder. One of the basic uses of a player handout (even other kinds of player handouts) is a reminder to let the players know what they should be doing. For example, if the players need to deliver a missive to Captain Barlio in Farhollow, presenting the missive as a handout helps distracted players answer questions that might come up after a distracting battle or two: "Where are we supposed to be going now?" "Oh, yeah, Farhollow." "Who are we supposed to meet there?" "Oh, yeah, that Barleycorn guy." (Never underestimate player ability to make silly names out of your most serious NPCs.)

* Roleplaying Tool. A handout can help with immersion in a game. If a player can physically hand a rolled-up missive to the GM with a line like, "An important message for you from the capitol, Captain," it helps the players stay in character. The same is true for other types of handouts; if the characters get split up, they know who has the map because it's the player who actually has the map in front of her. 

Incorporate more handouts, and your adventures will benefit!