Several times a week I work on a local TV show produced by my station. My favorite job is to run hand-held camera, which helps to break up the monotony of my day-to-day stuff which is quite repetitive. I know that many low-budget filmmakers will shoot entire movies this way to get a more "documentary" feel (or maybe they can't afford a tripod). There are lots of times when you'll have to go to your knees to get a shot, and you could really use something like Irwin's I-Gel Stabilizers that I found on Toolmonger. They will save your knees from a lot of damage, allow you to drop quickly, and only set you back about $25. Not bad if you do a lot of this stuff.
One tool I use on every shoot is a winding extension cord. These babies are great for giving you four plugs at the end of a 25' cord. They also make for quick striking, as the thing winds up to perfection in about 15 seconds. No more cord coiling! They are also compact, and can fit almost anywhere for nice portability. Found at almost any hardware store (or here, if you can't), these will cost you around $20, and are well worth it.
Another gadget I've used quite often, is a mini-DV tape rewinder. This seems like a no-brainer to save wear on your camera or deck, but I never even thought of it until I came across a clearance sale at Rat Shack that featured this Maxell model for $15. When I was shooting a TV show with three cameras and 6 tapes per show, I know this thing saved my tape heads from undo strain. They are harder to find locally, but eBay can help if no one else can.