The best saturn emulator at the moment is the "sega b-club" girigiri that can be downloaded from the sega b-club site. You have to pay/rent and download the games from them too. There is also a hack of it floating around the net.

Don't ask for it here though. Many games run full speed with sound. Very high compatibility rate.
The next alternatives would be satourne and ssf. Satourne has less compatibility but offers a little more speed. It's plugin system is still early and there aren't many plugins for it yet. Only the main authors plugins are available. Many other plugins may surface in the near future.
SSF uses software rendering. It's compatibility may be a little higher than satourne, but it's damn slow.