Scratch is an interactive programming project that lets you create animated items and share them with others. The editor is a downloadable program, so it’s not completely online, but there’s a pretty strong community aspect to the website anyway. It has the feel of an educational experiment about it, but will probably keep an 8 year old happy for a while, once they understand the interface. Worth a look.
Scratch is a new programming language that makes it easy to create your own interactive stories, animations, games, music, and art — and share your creations on the web. Scratch is designed to help young people (ages 8 and up) develop 21st century learning skills. As they create Scratch projects, young people learn important mathematical and computational ideas, while also gaining a deeper understanding of the process of design.