Down Tube Shifters have such a great uncluttered, old school look about them, and the short housing-less cable runs seem to provide crisper shifting. The Bike Hermit's A. Homer Hilsen sports a pair of down tube shifters. He finds them most excellent for touring.
The action of these shifters is smooth, light, simple, fast, and precise. These work in friction mode only....no indexing. They work with any derailleur (except SRAM), and freewheel, cassette, chain, etc., so you have the peace of mind that comes from knowing you'll never be midway between gears and powerless to do anything about it.
You will need a couple of shifter cables with these and a short piece of housing for the rear derailleur. Order them with cables and housing if you don't already have some.