This site is a few years old now, but it is probably the site that I am most proud of, and certainly that has had the most work done on it!
The concept is basically an e-learning site - the initial focus was on getting children to take more of an interest in science, while improving their analytical skills. I took the original concept, and wrapped it around a membership based site, with an eye towards user interaction - "liking" other users answers, customising the colour scheme, progressing through levels depending on the points accrued on the site. It needs rebuilt now, (the project was started 8 years ago) but at its peak there were hundreds of users interacting with the site on a daily basis, with surprisingly few problems!
The site was built using php and mySQL, with a couple of open source plug-ins for the secure registration/login system. I particularly enjoyed the text to speech function that allows the robot to speak, much fun was had in development getting him to say inappropriate phrases!