I went to Wal Mart (other stores have similar kits) and for less than $10, I got one heck of a nice tool kit. Pliers, wire cutters, standard and metric sockets, standard and phillips screwdriver etc... In addition to this, I keep a little bag with small and medium hose clamps, spare fuses and plugs, my plug wrench and a few "necessary" items that will always bail you out, electrical tape and wire ties. As for the tools, the have a nice box that keeps everything in it place, it stores "flat" so it doesn't take up space, and it seals rather well to keep out moisture. You also don't feel too badly if you drop one overboard ore loose something out of the kit.
One last thing that some people have trouble with, make sure that you have a lug wrench that will work on you trailer wheel lugs. I ran into this situation. I now carry a four way wrench in both of my vehicles.
Bass Pro Has nice Waterproof Bags which will also fit a spare Prop and misc stuff. I am definately overpreparred and also have a Tow Rope/ Your Anchor Rope will work, Flashlight, Spare Bulbs (including Bow and Anchor Light) First Aid Kit, Test Light, Elec Tape, Wire, and a few Extra Electrical Butt Connectors and Misc connectors. Glad to say I have had almost no problems over the years, but always seem to have something to help someone else. Its interesting to see what others carry on board. Got Jumper Cables in your Vehicle? Will your Jack for your Car also lift your Trailer in an emergency? Bug Spray, Hand Towel, the list goes on and on.