mod_accel and mod_deflate
mod_accel is a reverse-proxy caching module (like mod_proxy, but it has many more features). mod_accel saves the backend's response to a temporary file if its size is bigger than the specified memory buffer, so the backend is freed as soon as possible. mod_accel saves the whole client's upload completely before connecting to the backend. mod_accel can use busy locks, i.e., if the cached response to some request is missed or expired and another Apache process has already sent the same request to the backend, then the current Apache process will wait until another process's response is ready and then will get it from the cache. mod_accel can take into account specified cookies when caching the response. In addition to all this, mod_accel has many directives that allow flexible configuration of the various buffers, the caching parameters, and the timeouts. The documentation included in the tarball is in Russian only, but there is an incomplete English translation available in The Wayback Machine. There is also German translation.
mod_deflate is the module to compress a response using the gzip or deflate methods. It is similar to mod_gzip but compresses the dynamic responses without the temporary files. The documentation included in the tarball is in Russian only, but there is Babelfish English translation. This module has no relation to Apache 2 mod_deflate module.
Both modules have been in use on several heavily loaded Russian sites for more than 2 years. There are some English links: www.fastmail.fm success story by Jeremy Howard and the installation example by Philip Mak.
(C) Igor Sysoev