So let's talk about character portraits.
I've been using character portraits at the table for nearly 4 years and determined that they are among the most versatile and indispensable items to help players visualize and remember the most important parts of the game: the characters that make up the world.
I started with a Smallville game - a game where a web of characters is literally the first thing created in play. I used Creative Commons licensed portraits from flickr, photoshopped nice bold text onto them showing their names, and when I put them out on the table, people immediately recognized them. For annoying or dastardly characters, all I had to do was put them out and the players would immediately start groaning and responding. It was thrilling to see how just a visualization could help them get into the game.
Even more importantly, it helped them stay in-character. No longer were they saying "I go and talk to that one guy", they would call him by name, remember his face, refer to him in dialogue. This simple aid improved my groups' roleplay immeasurably.
This free product (free?! what?!) is 108 character portraits, in .jpg format, licensed under Creative Commons. Unlike the color photos I got from flickr, you won't need a photo printer to use these - they're bold line drawings and well-shaded with good contrast, so they will look good on your regular old normal printer. The portraits are divided into sections including "cyberpunk" and "scary" and "elf" and so on, to make it fairly easy to find the portrait you need.
As a free product in a category that has been one of my most proven methods of improving play, I absolutely give this product my highest rating. You have no excuse not to try using these! The only game that they won't improve is a game where characters do not play a central role - and that type of game is awful anyway. Put more characters in your game, people care about and respond to characters more than they do to animals or the weather or whatever it is you have in your game other than characters. And use this free product to start!