bikejournal.com has an ongoing thread called Commuter Century Cycling Challenge (CCCC). One work day where a bicycle is used to commute counts as one bike commute day, regardless of how far or how many other stops or side trips or round trips are made during the day. This keeps it simple. Participants in CCCC complete bike commutes during the year to pass various milestones that are tracked by date. 25, 50, 100, 150 and 200 commutes. It's a great group of people from all corners of the country and many other countries. The CCCC list is updated every week or so by a thread moderator, and it's amazing to see what so many are able to accomplish. Most people post updates one or more times per week.