A pot of beans. Can be made ahead of time and frozen. Can be made without any animal products so as to be acceptable to both vegetarians and vegans.

i also am a big fan of grains. i've made this quinoa salad from epiciurious here for a few bbqs, and everyone always loves it. double the spices. and you can sub asparagus or green beans for the zucchini. good luck!

These Sesame Ginger Sugar Snap Peas make a really tasty side dish. They are super easy to make AND they can be made ahead of time and served at room temperature.

http://www.atthefarmersmarket.com/2010/05/sesame-ginger-sugar-snap-peas.html

Pimm's Cup is a great drink for a crowd.

This is a fantastic salad: http://www.epicurious.com/recipes/food/views/Roasted-Potato-and-Okra-Salad-232318

Succotash is a great hearty salad too (lima, corn, tomato).

Vodka slushes are perfect for summer and go a long way:

Stir together in a large tupperware:
6 oz frozen orange juice concentrate, thawed
12 oz frozen lemonade concentrate, thawed
12 oz frozen limeade concentrate, thawed
1 cup sugar
3 1/2 cups water
2 cups vodka

Freeze, tightly covered, for 48 hours. Stir occasionally.

To serve, spoon about 1/2 cup to 3/4 cup of slush mix into a cup, and pour Sprite (or Diet Sprite) over it to cover. Icy cold, sweet, delicious!

How about pitchers of spiked lemonade, lime rickey, or sangria?
I've had good luck with this punch too (complete with lemonade ice ring): http://www.foodnetwork.com/recipes/paula-deen/southern-comfort-punch-recipe/index.html

I have made this many times, everyone always loves it!
The original, in my hand, is clipped from the BH&G August of 1990 issue. It says it serves 16, and doubled, 30 (?). I usually cook my own beans, and use brown rice instead. It's awfully good.

http://www.bhg.com/recipe/salads/two-bean-and-rice-salad/

This punch is very good:

http://www.marthastewart.com/recipe/southern-comfort-punch

I actually bring soba noodles to bbqs a lot and they go really fast. can make these ahead:
http://abcdsofcooking.blogspot.com/2009/08/otsu.html

Also these Indian tea sandwiches were a hit. Can't make the whole sandwich ahead, but you can make the chutney/cream cheese mixture the day before:

http://abcdsofcooking.blogspot.com/2010/06/indian-tea-sandwiches.html

Two words (or is it three?): Cous-cous salad. Make ahead. Good cold or at room temp, infinitely variable with any veggies you like.

Steep the cous-cous. Add cut up veggies (raw/roasted). Dress with olive oil, lemon juice and go hog-wild with herbs, lemon zest, feta cheese.

Israeli cous cous with kalamata olives, cherry tomatoes, feta cheese and toasted pine nuts. Make a lemon vinagrette dressing.

Check out Mark Bittman's series - 100 Simple... For example : http://www.nytimes.com/2007/07/18/dining/18mini.html?_r=1

If you like kohlrabi, Serious Eats has this recipe for Kohlrabi Remoulade:
http://www.seriouseats.com/recipes/2009/09/the-crisper-whisperer-what-to-do-with-kohlrabi-remoulade-recipe.html

Essentially, you just trim and peel the bulb then grate it into little pieces, mix with some mayo, mustard, vinegar, garlic and spices. I made it last night with kewpie mayo, yellow mustard and rice vinegar--delicious. Sort of like coleslaw, so familiar, but just different enough. Next time, I will probably add some carrots into the mix.

alcohol idea:

sangria - i used 3.99 white lambrusco mixed with lemon mineral water and cut up strawberries plus a splash of hard alcohol (i had peach schnapps lying around.) so delish and easy.

This wheat berry salad is always a hit and can be prepared in advance:

http://tastespace.wordpress.com/2010/03/07/wheat-berry-salad-with-almonds-and-spinach-in-a-citrus-dressing/

The miso salad from 101cookbooks is pretty cheap, can be made in huge quantities, and is universally loved by everyone I've made it for. It also fits the bill of "unusual," although it does share some similarities with the ubiquitous asian-ramen-noodle-slaw-type dishes that always seem to show up at cookouts. Still, I think it's much better than those. Here's the recipe:

http://www.101cookbooks.com/archives/chopped-miso-salad-recipe.html

I like to swap out the tofu with roasted mushrooms!

I got this recipe from an old "Cheap Thrills" comic strip. It's great on its own or alongside grilled meats.

Grilled Pineapple Salad -- 4 servings

Dressing:
2 peeled kiwis; 1/3 c. vegetable oil; 2 T. balsamic vinegar; 1 t. Dijon mustard; 2 t. honey; 1 T. lime zest; 2 T. diced lime pulp

Salad:
1 whole pineapple; washed lettuce leaves; cottage cheese (optional); kiwi slices for garnish

In food processor, puree first five ingredients for dressing. Add in lime zest and pulp, then set aside. // Peel, core, and cut the pineapple into 8 long spears. Brush the bbq grill with oil and turn to high. Bbq spears on one side for 3-5 minutes. // When done, arrange spears on lettuce-lined plates (or platter). Spoon dressing over pineapple, garnish with kiwi slices. Serve with scoops of cottage cheese, if desired.