Usability 
loband is a non-profit project whose goal is to make complex pages usable over low bandwidth connections in underdeveloped countries. It also helps make sites more usable on PDAs and phones. While not billed as a mobile transcoding proxy, I've included it because it shares many of the characteristics of one.
I've found this site unreliable in the past and, in fact, it was down for several days while I was testing. Loband doesn't split pages that are too large for the device. Loband replaces images with an "[i]" link, clicking the link will attempt to display the original image without any rescaling or image conversion. Loband also unnecessarily removes text and background colors making everything black text on a white background. Both my test pages were too large to load on the Openwave, Motorola, Sony Ericsson and Nokia emulators. Using the Access smartphone emulator, which can load extremely large pages, I was able to browse this blog and most of the Southwest site. When I got to the secure page where I would enter my credit card details, loband redirected me to the real Southwest web page which was not usable on the mobile browser.
Screenshots:
