As part of an effort to thwart future ISP infractions, á la Comcast, the Electronic Frontier Foundation (EFF) has released Switzerland, an open source software tool for "testing the integrity of data communications over networks, ISPs and firewalls." If you've been following Comcast for any amount of time over the past year or so, you know exactly what that means.
It's available for download now, but be warned: this is an alpha release command line tool (read: newbies need not apply just yet). Serious code monkeys are encouraged to download and kick the tires a bit to see if it does as advertised. Mainly, that means detection of packet modifications or injections as they travel over IP networks. Past culprits in this category include anti-P2P tools from Sandvine (Comcast vs. BitTorrent), AudibleMagic, advertising injection systems like FairEagle, and censorship programs like those used in China.
Here's a little bit more on Switzerland from the EFF:
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 [...] Switzerland is a much more sophisticated successor to the pcapdiff software that we released last year. It automates many of the things that had to be done by hand with the earlier code.
One advantage this architecture has over other network testing tools is that it can spot arbitrary kinds of packet modifications in any protocol - it doesn't assume that the interference comes in the form of TCP reset packets or web page modifications, and it isn't limited to BitTorrent or any other specific application.