My computer and internet connection?
I learned a ton back in the day when I first found the old BloodBrothers site when Dan was still on there. Then I bought the DVDs, and have a constant access to a wealth of knowledge here.
I owe several of the deer I've taken over the past couple years in large part to having my GPS. Being able to draw up a "going in blind" route on the computer, transferring it to the GPS along with Bing Bird's Eye and topo lines, is an awesome tool. The buck in my avatar picture I shot after studying topo lines on the GPS in the stand after a weather switch, and making a move midday. Not to mention, that little piece of equipment has gotten me out of some sticky situations in river bottoms in the dark.