The CHICKEN team is a big fan of dog fooding: the wiki, pastebin, egg documentation search (which is yet to be updated to C5) and our infrastructure sysadmin stuff in the background is all written in CHICKEN.

It's not a high-volume application by any means, but I wrote an online testing program (multiple choice, etc.) in Chicken that's been running at our university for about twelve years. It was meant as a short-term, interim solution until the bugs in our 'enterprise' testing system were fixed... but it still finds some use today. (I just took a peek at the database; we've had about 250,000 test submissions over the years. Not bad for a little Scheme app.)