Usually sour cream, sharp cheddar and green onion. I especially like shredded Monterey Jack or Cabo brand Habanero cheese if I have that on hand instead. Sometimes though, depending on the chili, I don't want anything creamy in it, I just want to enjoy it as is, so I may just go with a little green onion or cilantro instead for a little color. If the chili has a lot of beans in it, I don't mind the creaminess of sour cream and cheese but if it's got a lot of high quality meat in it, and expensive spices, I hate to drown those out with dairy products.
Some crispy match stick fries might be fun too, now that I think about it, or fried plantain chips.