Semi-standard BYO list for gatherings (YMMV):
Sleeping gear (sleeping bag, nesting materials, and so forth)
Warm things to wear or wrap up in, even if it's not cold
Towel, washcloth, toiletries
Portable tape player or minidisc recorder
Your own folding chair (bring a sunhat)
Whatever day-starter you are addicted to (coffee in singles-packs, for example)
Snacky and sandwichy stuff, fingerfood varieties, and a disposable platter to set them out on. A platter or other serving item that has a see-through cover is ideal. If it isn't disposable, be sure you wash it yorself and get it back out to your car to keep the kitchen messes under control.
Cheeses cut up in cubes are always popular, with some crusty bread to tear off in chunks
Entrees: If you bring something that's pretty spicy, LABEL it when you set it out
Don't be offended if you bring something and it isn't set out right away-- the host may have a plan, and no one wants to see exactly the same buffet three days in a row. (Also there may just be TOO MUCH food, and believe me, your host will appreciate having an excuse not to cook for a coupla days after you all leave!) You might consider bringing some of it frozen in a container that can be used to nuke it when needed and/or left behind. Those semi-disposable Glad plastic boxes are GREAT for Gathering foods.
Bagged ice, paper goods and tinfoil are usually appreciated
REALLY HELPFUL-- if you are not sure what to bring, or if the trip will be long and you are concerned about stuff not staying fresh-- offer ahead of time to be the one that goes to the store once the rest of the stuff arrives, to fill in whatever seems lacking.
JOBS THAT ALWAYS "NEED DONE"
The whole food thing works effortlessly AS LONG AS THE KITCHEN IS POLICED EACH NIGHT BEFORE BEDTIME, including laying out the breakfast options or hanging signs on them in the fridge. Periodic preening of the buffet is helpful too, consoolidating stuff or adding new stuff, and icing anything that looks like it's on the MUST REFRIGERATE list. (That's where the bagged ice comes in-- put a pile in doubled plastic bags on top of a towel, and set the food container on it.)
The outdoors can get messy too. When outdoor picking ends, pickup.
Someone will need to police the smoking area too.
Someone will need to be sure a recycling area is set up and someone may need to deliver the items to the recycling center when things end.
Watch for people needing certain chairs, etc., and/or people with special physical needs. If you spot a need, hop to it, unobtrusively. Bring in someone's gear for them, or help them set up their sleeping spot, and so forth. These Catters can be pretty creaky and not say a word about it.
Most Mudcatters will do all this instinctively.... but this is a starting place if this kind of party is new to you. And it is always helpful-- if you plan on bringing something specific or doing one of these jobs-- let the host know ahead of time. The mental to-do list gets quite long.