This looks very interesting -- a MR-16 fluorescent bulb ! They claim 2.5 watts at 12v, with the light of a 20 watt incandescent.

It's sold for use on sailboats. Price is $24.95

http://www.sailboatstuff.com/lt_G4_MR11_MR16.html Scroll down to the bottom of the page.

The same page also has Xenon MR-16 bulbs in 10watt and 20watt, which look interesting as well.