Besides driftwood you can also use Mopani and Wellaby wood. They are both an African wood that has been sandblasted to produce a light and and a dark side. Both of these will sink on their own when placed into the tank. Just like driftwood though, they will stain the water. Some people don't like this effect, I love it.
I'm not sure if Mopani and Wellaby wood are actually the same wood being sold under different names or not.
Bryan