The Electronic Frontier Foundation has released an open-source, cross-platform program designed to track your packets and determine if your ISP is throttling your connection to torrents, VoIP, and other legal, high-bandwidth consuming communications. Called "Switzerland" and licensed under the GPL, it's very much in an alpha state and is only a command-line tool at the moment. Also, you're going to have to compile it yourself--that's not the most challenging task, but this isn't a simple self-extracting app.
According to the EFF, Switzerland works by spotting IP packets that have been forged or modified between clients, informing you of the change, and providing you copies of the modified packets. "The software uses a semi-P2P, server-and-many-clients architecture. Whenever the clients send packets to each other, the server will attempt to determine if any of them were dropped, forged, or modified," says the Switzerland Web site.
As far as usage goes, the EFF says that Switzerland is compatible with NAT firewalls, although some NAT firewalls may have to be disabled to test the ISP in front of it, because of the modifications that some firewalls make to packets.
I do wonder at the logic of the name, though. Referencing the "neutral" country is cute, but what's going to happen when somebody tries to find the program through a search engine? Googling "Switzerland" returns 234 million results, give or take.
Anyway, Switzerland is not the first packet-testing program around. What is special about it, though, is that unlike, for example, the plug-in for the Vuze/Azureus torrent client, Switzerland isn't tied to any host program. The open-source license, combined with the backing of a visible group like the EFF and the building awareness in both politicians and the general public of what Net Neutrality is about, could have serious ramifications for combating false promises of Net Neutrality from ISPs like Comcast.