This site is a few years old now, but it is probably the site that we are 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. We 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 6 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. We 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!