(That one-page J interpreter is sometimes called the "J Incunabulum".)
See e.g. http://news.ycombinator.com/item?id=1458016 http://news.ycombinator.com/item?id=266982 http://news.ycombinator.com/item?id=697501 http://news.ycombinator.com/item?id=2176980