What's the first thing that comes to your mind when you think of centerpieces? Typically, the answer is flowers and there's nothing wrong with that, but do something different with them. Ideas include vases of different dimensions, different floral arrangements (don't do the same thing on each table), arrange the flowers in a shape other than a circle or square- like an eiffel tower. It can be something as simple as adding a few drops of food coloring that match your colors to the water, add pearls or fruit such as lemons, oranges, and limes (great for spring and summer weddings) to the bottom, add candles around the arrangement.
Shells and sand- try colored!- are perfect for a beach themed wedding.
Interactive centerpieces are great, too. Think about doing a simple cake- a different flavor for each table?- instead of having sheet cakes back in the kitchen.
A money saver would be to double the centerpieces as favors. Imagine small, potted plants, or picture frames with different pictures in each.
For a real conversation starter and something guests will talk about long after the wedding, how about suspended flowers?