Apt-Cacher NG is a caching proxy.
Editor: A caching proxy. Nothing really innovative. See documentation of Apt-Cacher to learn what is good for.

Why should I use it?
AC-NG does not require an interpreter or huge runtime library. It does never fork after server startup, it does not Create Flag files, Flock() files or Similar fun.Instead, it uses native system functions (mmap, sendfile) to operate with few overhead.And last but not least, there are some goodies like the interactive report page, see below. you can free download Apt-Cacher NG 0.3.3 now.