+1 on staying found. Don't wait until it's too late to stay on top of knowing where you are at all times. A GPS is great for that but a map, compass, and the knowledge to use them is a must have IMO.

I use a GMRS radio to keep in touch with my wife while camping and I've never (despite lots of monitoring and scanning) picked up anybody using within range but us. I keep one handy for SAR work too to monitor the air waves...it's light weight and you can never be sure who might be listening. In reality though you're better off with a good whistle.

Blast had some good ones. I'm a SAR volunteer and I've NEVER considered leaving pictures of my gear and a boot print. Mantracking would be SOOO much easier if we went out knowing what their boot treads looked like!

You're going to find different lists of the 'top essential' items. I feel it's best to be familiar with several as they're all very similar. Many of the items are tiny and easy to carry (like a firesteel or a signal mirror) so there's no excuse for not having them handy. The Boreal Instutute has some good lists I keep around for when I'm trying to lighten my load to remind me which items are worth the weight.

http://www.boreal.net

My personal must have is a gear repair kit. There is nothing worse than being days into the hinterlands and having a pack strap break or the bottom tear out of your pack or a shoe blow out or you get a hole in your tent (the list goes on). We don't all hike with brand new shiny gear and as it wears this becomes even more important...though accidents happen an no gear is impervious to damage. My kit is fairly beefy but that's only because I like to be the boy scout always prepared hero type when I'm out with friends. I can't tell you how many sleeping pads I've patched with nothing more than a mini glue stick and a mini bic. Even if gear repair is nothing more than duct tape and some needles and thread it's better than nothing.