I've got two sets of these and they work great. Friction only (no indexed shifting) but almost never need adjusted once set up, which is so easy I did it successfully on the first try and I'm probably the least mechanically inclined person posting on these boards. Plus, they are REALLY, REALLY inexpensive compared to the other two choices discussed in this thread.

http://www.rivbike.com/product-p/sh3.htm

I was intrigued with bar-end shifters for the longest time but now that I've ridden a couple of bikes equipped with them, I'd ride a single-speed before I'd spend time on a bike with bar-ends. From an ergonomic standpoint, STI cannot be beat. If you are mechanically inclined and willing to learn to adjust them, they would be my first choice.