As a player: Dice, a clipboard with my character sheets, extra paper, and any notes. A mechanical pencil, eraser if the pencil's eraser is gone or worn out. Maybe I'll bring a couple of drinks to store in the fridge for later.
As a GM: The main rulebook, a small notebook with NPCs, my world notes and maps, and the notes for tonight's adventure. Maybe a separate folder with pics and handouts. I usually will not bring more books than the main rulebook; very unusually, I'll bring a single supplement if the adventure might call for it -- for instance, if I'm running an Eberron game in Sharn, I'll bring the PHB (to look up NPC spells if I have a question) and the Sharn book. Same clipboard/dice/pencil I bring as a player. Almost always some water or drinks for later; I get thirsty when I GM.