Sometimes it's just very handy to have a generic set of tools at your disposal. You never know when you'll need to repair a set, build a prop or fix your car right there on location. I like Bucket Boss' Duckwear Tool Roll which gives you the basics and rolls up into a handy bundle that can be tossed into an car trunk, crate, or backpack. It's also a good deal at $15.95 and will come in handy for many shoots to come and should last as long as you do.
Have a Ball
While I'll never give up my Sennheiser ME66 shotgun mic for recording dialogue, I really wish I had a good desktop mic that plugged directly into my laptop. Blue Microphones makes such a thing dubbed the "Snowball" that fits all requirements above and even can switch between several different pickup patterns depending on what you need. It would be perfect for a looping mic used in conjunction with something like the "Porta-Booth". At $139 it's not cheap (good audio never is), but I'd still consider it inexpensive. Ken Stone has a full writeup here.
Better than a Credit Card
Every video camera is able to charge its own batteries, but this means your camera must remain dormant during the process. What if you need your camera to operate AND charge batteries? What if you can't wait for a full charge cycle? A speed charger is your answer and I recommend the Lenmar brand. It will charge several types of batteries faster than your cam can (from an AC outlet or cigarette lighter in your car), and won't overcharge, which should give your batteries longer life. I purchased mine from Battery Barn, which is now going for $65. I don't remember paying that much, but it looks like they now come with an attachment for AA/AAA charging as well. No fair!