Shimano makes their Flight Deck Computer to work with their brifters, it shows you your gears and tooth counts and can calculate cadence without a cadence sensor. I have one on my Salsa, and like it a lot....
http://bike.shimano.com/catalog/cycl...=1153414380006
B'Dog
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2006 Gary Fisher Marlin MTB
2004 Salsa LaRaza aka "Mmmm, Blue Salsa" (853 steel is real, baby!)
2003 Kona Humu Humu Nuka Nuka Apua'A Singlespeed - Gone, but not forgotten
1998 Diamondback Outlook MTB
1990 Centurion Cavaletto 'Gaspipe Beater' Roadie
1978 Volkscycle 'Frankenbike' CroMo Tourer