MyHair's Journal: Mirroring FTP Proxy?

Okay, here's what I want to do: when FTP-installing a linux distro (Suse, Debian or Slackware usually) or Cygwin, I want to download only the files I use but have them available locally for reinstallation or setting up another PC. I have multiple PCs available. Here's a sketch:

[Public linux/Cygwin Mirror]
        |
[My proxy/mirror/FTP server]
        |
  [PC getting linux/Cygwin]

From the installing PC I'll specify My proxy/mirror server as the FTP source. The proxy/mirror will mirror the directory listings of the public mirror and download requested files keeping a local copy. Of course if an up-to-date local copy exists it will simply serve it to the installing PC.

The effect will be that I'll have a partial local mirror of the install files, but these are the very files I'm very likely to use again soon. So if I install from another local PC it will be uber-fast. Also my local partial mirror should be easier to fit on a CDROM or two for carrying to/from work.

I expect there's software out there that already does this, but I haven't found it yet.

Does anyone know of a free software package that does this?