external flash on Fuji X-s1
If you want TTL functionality you can use either the Fuji EF-20 or the EF-42. The EF-20 is a good flash with bounce capability and is a nice compact size. The EF-42 is a more traditional dSLR style bounce/swivel flash gun and is a lot more powerful - it all depends if you want to carry around this beast on the top of your camera!
As far as I can find, there are no 3rd party TTL flash units available for the Fuji system. If you don't care about TTL then the Yongnuo 560 II (at about £50 on Amazon/eBay - don't pay more) is a fantastic flash for the money.