Here is a link to the IPCop site:
http://sourceforge.net/apps/trac/ipcop/wiki
Here is a link to the Copfilter site (which is currently broken as of yesterday unfortunately):
http://www.copfilter.org/
If you have a look at the IPCop page & go to the Add-ons link, you will see quite a list of additions.
The Proxy add-on that I couldn't remember in my last post is Advanced Proxy:
http://www.advproxy.net/
I'm not a lot of help with the details of IPCop/Copfilter. I set it up about a year ago, in the process I learned a lot about it & associated stuff, after which I haven't really had to touch it. So I've forgotten most of what I knew.

I just know it works great, & I'm very happy that I went to the trouble of installing it.
By the way, I recently obtained a gizmo that allows me to measure the power usage of anything that plugs into a power-point. I could put in what I have to pay per kWh ($0.19.62 cents Au) & it calculates the costs per hour, quarter & year.
The old PIII 733 that I use as the headless IPCop box, costs $55- Oz / year to run. Which isn't to bad I think.
As far as Copfilter working like Squid/Sarg is concerned? I don't think so. Copfilter provided an additional 10 services for the IPCop user. Of those 10, I only use 4, being: the Monitoring Utility; HAVP; Privoxy; ClamAV. I don't need ClamAV, (only Linux & OS X here) but because it has no effect on the speed of our internet access, I say why not.
The links I provided earlier in this post really should answer any questions about IPCop & the Add-ons.