The city of Copenhagen, Denmark, our Best of Green 2009 winner for "Best City To Be a Cyclist In", has installed what I think is a totally awesome cyclist counter ("cykelbarometer") in front of city hall (it also has an air pump if you need to inflate your bike's tires). The two numbers on the counter are "total number today" and "total number this year so far", according to our friend Mikeal from Copenhagenize. The city will give a Biomega bike to cyclist number 500,000, and since there are about 12,000 cyclists that pass by on a week day (about 25k both ways), it shouldn't take too long for that number to be reached.
How does it work? From Copenhagenize:
There is a 'sensor line' in the asphalt on the bike lane a few metres in front of the counter which registers the cyclists. There is a SIM-card in the counters so the information is automatically sent to the City of Copenhagen's Center for Traffic. The counter only registers cyclists on this side of the street, not the far side, so you can double the numbers up to see how many cyclists use both directions.
I think more cities (and not just in Denmark) should have cyclist counters. Information about how many cyclists are around can be a powerful tool for those who argue for more bike paths and more cyclist-friendly streets, and it can probably also nudge motorists to give biking a go ("wow, 12,000 people rode bikes on that street today, maybe I should join them...").