An iPhone app, explained and demonstrated well here:
I love this app and enjoy making, say, going shopping aurally weird and arty and challenging. It's cool to be connected to your environment and disoriented at the same time. A bit like being crazy, but you can turn it off. I was walking around a bookstore with "Echolon" playing, and it picked up snippets of conversation and echoed and transformed them in a highly amusing way.
ADDED: I realized that running this app into your own ears is like imposing a "Harrison Bergeron" program on yourself. Are some children smarter than others? Let them listen to the teacher through "Echolon."