All legal docs etc that you can find. Especially important, things like birth certificates, passports, red book, etc.
Utility bill or two in your name. Preferably one from each utility in case you forget who you are with. (I couldn't remember whether we were with British Gas or Eon when it came to cancelling my part of the contract)
Bank statements. Building society books. Child Trust Fund etc details. Bank cards. Get some cash as soon as you can too. Mobile phone. (Switch it off and use phone boxes though or go to a newsagent and buy a cheap sim card on the same network to call your parents). Whatever you do, don't answer if he calls. It's going to be a shock for him but it's not going to harm him to be away from his DS for a night, as that's probably the first guilt trip he will roll out. You can talk to him about DS in the morning. If you can get your parents to call him tonight on your behalf so much the better.
Any computer/laptop which is yours if it has stuff on it and you can carry it. (Don't forget the charger) If not, WIPE YOUR INTERNET HISTORY BEFORE YOU GO. Download a little program called "CCleaner" and install it (takes seconds) then tick every little box on it and run the scan. It won't delete any documents or programs, but will delete history, lists of recent files accessed, saved passwords, etc. If you can't carry the computer but you want it, open up the case with a screwdriver and just take the hard drive. Should be a little box slightly smaller and thicker than a DVD case.
DS's favourite toy/comforter if he has one. Some of his books for the journey and to keep familiar. The pillowcase off your bed (will smell of you) and the sheet from his cot (again will smell familiar) Any small sentimental things you can carry (memory card from your camera?) if you don't think he will let you come back for them. Don't worry about toiletries, nappies, milk etc - take enough for the journey, you can buy more when you get there.
If you have more than one buggy take the one you can fit the most stuff on to carry. If you have a sling or backpack carrier even better - carry him in the sling and put the stuff in a big carrier in the buggy. Make sure you can carry/drag this bag if DS gets too heavy to be carried or he wants to sleep in buggy.
If you are stuck for somewhere to go, don't hesitate to call women's aid. (0808 2000 247) They can put you up for a night, he doesn't have to have hit you.
Good luck. I think you are so brave. (Even if you don't feel it!) I'm sorry people have been discouraging on this thread. If you're thinking of leaving I don't see how that can be OTT - nobody gets to that decision overnight.