Will,
A few additions I have is a note pad and a pen/pencil. A training diary to keep track of sets and reps to see progress and cue me when to change my work-out. More importantly I use it to keep track of sets. The little "ticks" on the page don't lie about how many reps/sets I've done.

A big one for me is my MP3 player and ear-buds. If I'm not into the music being played I've got my own - not to mention it keeps me from "socializing" when I should be working. Save the chit chat for the post workout shake bar! (Make sure you carry a spare battery!)

Also gloves! I have a few pair - some that work well with particular exercises (dumbbell vs bar etc).

Post workout meal - what ever it is! I use your "roll your own" creatin, gatorade, protein powder, water mixture and sip it throughout the workout.

Water bottle - nuff said!