advproxy   - squid compiled with all kinds of useful options not in the standard IPCop version, and gui enhancements to get at them.

urlfilter - squidguard with a reasonably useful gui. No, scratch that, the gui is pretty decent and flexible, but I've been spoiled at work by exposure to SurfControl and BlueCoat.

Unofficial addon server for IPCop - easy way to add all sorts of extra functionality to IPCop; don't currently have any modules installed through it, but it's still installed from testing the squidguard package available for it (not flexible enough for me, but dead easy to install and configure).