Shimeji

A computer program that creates animated characters who walk and crawl along the sides the monitor, perform a set number of animations, and multiply. Popular among Japanese anime and game fans, who create shimeji programs for popular characters. The name comes from a fast-growing breed of Japanese mushroom.

I left my K-ON shimeji running, and when I came back, the entire screen was full of Mio and Yui rocking out.