I was testing some gear at the local rodeo and had a chance to kit my M9 out with the Zeiss 25mm f2.8 Biogon. This is the gem of the Zeiss lineup and one of the best selling lenses at my store.
For years rangefinder lenses went from the Leica 28mm down to the 21mm f2.8. Then Leica came out with a 24mm and Zeiss brought out the Biogon 25mm. You should use the viewfinder on the hotshoe if you want to frame things properly, although with the digital camera you can always shoot and chimp.
When I travel I usually take along the 25mm because it's such a useful focal length and gives a reasonable spread from the standard 35mm if you don't want to carry a whole bag of toys.
These two images were done on the M9 and the Biogon 25mm.
My retail store for rangefinder photo supplies is http://www.rangefinderstore.com