There are lots of ways to get creative with centerpieces for Halloween parties. You can cut the upper half off a pumpkin and then fill it with candy, flowers (preferably black ones!), or punch (put a bowl inside it). The pumpkins by themselves can be the main focus if you paint them black. If you have a vase of flowers cover them with plastic spiders and hang them on webs over the side of it. Rubber stamping spiders all over your glass containers (use Windex to clean them off) will send a chill down your guests' spines.
Candles can always go in the middle with cobwebs spreading out from the candlestick. If you want to go for the ghoulish gusto you can drip hot wax from a red candle onto lighter-colored ones. It'll look like a bloody accident took place nearby!