A friend and I built a website that allows users to log running workouts and then run analytics on those workouts. So, for example, if I ran 5 miles, I can go to a calendar, open a form on that day, and enter my workout details (mileage run, time it took, how I felt, shoes I wore, etc).
It also has social aspects to it... so I can share my workout with my "friends" or my "teammates" (a group). It also has the activity stream idea built in, so friends can talk to one another, share workout ideas, etc. I want to have an events section that lists races my users are going to run and then allows others to sign up for those races and discuss their training for them, how they did after the event, etc. The social aspects are wide and varied.
The main draw of the site, though, is the ability to log a workout (manually as noted above, by connecting a GPS Gamin Watch etc). We can even map the run (using Google Maps or the Garmin API) and then run analytics to see how much faster you are on a given route today versus past periods.
Everything was built using cake, php, mysql, and all styles were css.
I have virtually no programming experience, but figured out css well enough over the course of a few days/weeks to be able to style the entire site. Its not gorgeous, but its not bad, either.