COINTELPRO, short for Counter Intelligence Program, was a series of secret FBI operations between 1956 and 1971. For the most part the program centralized on disrupting movements related to black liberation and black nationalism, as well as communist and socialist organizations, and to a lesser extent white supremacist organizations such as the Ku Klux Klan.
What I've found most interesting about the counter intelligence program is the fact that it's an actual conspiracy to commit illegal acts by a government agency, yet most conspiracy theorists never talk about it. Instead, they prefer to talk about non-existent programs or change around their actual purpose to be something sinister.
Once while speaking to a conspiracy theorist, I brought up COINTELPRO and he didn't believe me that it was an actual government program, even though you can note its existence on fbi.gov.
COINTELPRO used several methods of action against opposition, I use the Black Panther Party as an example of what happened in many movements, they are as follows:
Further acts of illegal surveillance, wiretaps, etc, all without a PATRIOT Act mind you - proving if they really want to mess with you, they don't need legislation.
I could go on and on, but what else is there to say? The FBI conspired with police and informants to disrupt New Left (and some far-right) movements meanwhile harassing, beating, and in some cases killing those involved.
If you'd like to read more on the subject, be sure to check out these books at your local library or bookstore:
You can pretty much look anywhere on the Internet and at most bookstores and you can find more information on it, but there is some false information regarding it out there. The program no longer exists, however some say (I'm not one of them) that it is still going on, but there's absolutely no evidence of that. I should note, however, that the FBI has always been involved with infiltration of movements, so just because programs like that exist does not mean they are a part of COINTELPRO.