This excellent Adam Liptak piece is worth a read. It's about the website http://www.whosarat.com. Whosarat claims to be the internet's largest database of informants and undercover officers. Pretty clearly legal, (as it's based on public filings) this seems to me a lovely side effect of the government's consistent attempts to make criminal records and court cases available online to everyone. Of course the government is looking for ways to crack down on it and it's use.
Strangely as of the writing of this blog post, the website seems to be blocked (leaving me to wonder whether XICOM Technologies which seems to be doing the blocking is about to be hit with a very large lawsuit) but here's the old interface: