After a long work week, many folks in San Francisco hop on their bikes on a Friday afternoon. It's a big bike ride called Critical Mass. Biking is not just exercise, but a chance to be outdoors on the open road. Self sufficient. Going green. It's about as far from work as you can get in a matter of minutes — and, of course, the iPhone has an app for that. B.iCycle GPS, which costs $10, makes sure you don't get lost and tracks your distance, speed, hills climbed and even calories burned.