The Boy Scout of America Boy Scout Handbook lists the following as Scout Outdoor Essentials:

Pocketknfe
First aid kit
Extra clothing
Rain gear
Water bottle, with water
Flashlight
Trail food
Matches and fire starters
Sun protection
Map and Compass

I've read various additions on internet Troop sites, including:
Whistle - I'll suggest a Fox 40 or WindStorm
Insect repellent
Toilet paper (multi-purpose paper)
Large trash bag - multi-purpose, but can also be used to hold trash :-)

I'm wondering if anyone would have additional suggestions - keeping in mind that these are 11-18 year old boys. Many troops prohibit butane lighters.

I was thinking of adding:
Brightly colored bandana - could be a neckerchief
Plastic signal mirror (Star Flash)
Parachute cord, two 25" lengths

I've also thought about including a few water purification tablets (Micropur), but these probably should only be used by older boys.