For my campaigns it's initiative cards. I make those myself, finding images of monsters, PCs, or NPCs we'll use in the game (usually using Google Images), scale them to the size of a normal playing card, print them off, cut them to shape, glue them onto the back of an index card, cut them out again, write the creature's name on the back, and cover it with Con*Tact paper and cut it out one more time, this time leaving about a 1/8" border around the card.
The end result is a picture the size of a standard playing card that shows the players what monster they see (and, like Legatus_Legionis above, that means I don't have to tell the players what monster they're fighting) and shows the other players what each PC looks like. When we roll for initiative I jot the results on a piece of scratch paper, then build my initiative deck in order. The top card shows whose turn it is; when that player/monster takes its move the corresponding card goes to the bottom of the initiative deck. If you hold an action, the card goes sideways in the deck so you can easily see there's still a potential "out-of-sequence" move pending. And best of all, it allows me to decide what all of the monsters and NPCs look like. If the Monster Manual has sub-par artwork for a given monster, I can find a different picture that better represents what that monster looks like in my game. And if I can't find anything, I can always just draw it myself on the back of an index card and build my initiative card that way.
Over the years, I have amassed four index card boxes of these things. I only ever made the cards I'd need for the next adventure but once built I had the initiative card available for future use. (And if something comes up where there's suddenly a need for an initiative card I don't have - usually when a spellcasting PC summons something we've never used before - a standard playing card fills in just fine for the interim.)
Johnathan