If you have the cash to spare, you can try getting a TTL flash combination for the camera. Two such options are (a) Olympus FL-40 flash with their flash bracket and cable; and (b) Metz flash with the SCA3202 adaptor for Olympus digicams. I've no experience with either setup, but maybe others here can fill you in. You can probably find lots more about these elsewhere on the Web.
Other than that, you'll probably hear a lot about the Vivitar 285HV as one of the best generic auto flashes for most day-to-day use, and this works fine with the C-2100UZ if you get the expensive Olympus FL-CB04 cable. Personally, I use a Sunpak 383 flash (which is rather similar to the Vivitar 285HV) and I'm quite pleased with the results I've been able to get out of it.