Transparent Proxies for Linux

[Hans Aschauer <Hans . Aschauer @ physik . uni-muenchen . de>]

Hi all,
I thought that the transparent proxy option in the linux kernel looks
quite interesting, but up to now I only found a http- and a vdo-proxy
at ftp://ftp.ris.fr/pub/linux/proxy (can anyone help my what a vdo-proxy
is?)
Do you know of other proxies which can be used in a transparent mode
(with linux) and which are free? Are there perhaps patches for other
proxy products?

TIA, Hans.