PeerGuardian is a List Blocker.
It uses a text file to block ranges of IP addresses. It is only as good as the list of addresses contained within this file.
This file can be edited, and now it can also be used by
uTorrent to block the same IP address ranges. (You can easily find this info out at the
uTorrent web site
It downloads list updates that are compiled from
iblock list
For more
lists you can check here
More PeerGuardian info
Many of you may have seen my previous threads re: being slapped on the wrist by my ISP re: sharing files that I may not have realized I was not supposed to

In both of these instances I did not have PeerGuardian running, yet most of the time I do.
If you are shifty eyed porn king, then know that PeerGuardian will also block many of these sites too.
DO NOT over indulge on too many lists... only use the ones you really need or you will find many of the sites you need to access are being blocked.
An alternative to PeerGuardian is Protowall by Bluetack
located here
Bluetack also makes a
blocklist manager here to help organize and maintain your blocklist and convert them to other formats. For instance, many Firewall products will allow you to add blocklists for IP ranges that you want to block....but you really need to be careful with this and make sure you update this list regularly.