Would love to hear this group's suggestions.
http://www.philzimmermann.com/EN/faq/index.html
I have done some crypto work for financials and trust me if you aren't using an RSA algorithm then you get lots of questions and notice.
The NSA can neither confirm nor deny that they have trapdoors/backdoor access into RSA encryption. But if you don't use it they get very anxious to know what you are up to. http://www.rsa.com/rsalabs/node.asp?id=2316
As the premier cryptographic government agency, the NSA has huge financial and computer resources and employs a host of cryptographers. Developments in cryptography achieved at the NSA are not made public; this secrecy has led to many rumors about the NSA's ability to break popular cryptosystems like DES (see Section 3.2), as well as rumors that the NSA has secretly placed weaknesses, called ``trapdoors,'' in government-endorsed cryptosystems. These rumors have never been proved or disproved. Also the criteria used by the NSA in selecting cryptography standards have never been made public.
They came down on Zimmerman to help dissuade others from creating more encryption algorithms. This could be because they have control over others or they simply want to limit the resources needed to break each type of encryption.
See Joseph Menn's book on Napster for the backstory behind MP3 file sharing (many of us w00w00 folks were involved, including guys like "minus", who stole the Winamp source code off my friend Ian Rogers' laptop).
Other example I can think of - anakata and The Pirate Bay, which is sort of mainstream, I guess (!)
The original SATAN scanner from Dan Farmer; the concept, though. Bad guys had a bag of tricks that they'd use to get in. He compiled a list and built an automated tester of those tricks.
Some of the concepts that evil hax0rz/crackers used to obfuscate code in viruses have migrated into the mainstream as a method of protecting intellectual property.
There's an entire litany of examples where initially subversive technology became useful for defending against the same. The first vulnerability scanners were written to help attackers target vulnerable systems. They were quickly repurposed to help people find and protect vulnerable systems.
The whole "evil hackers" thing is kind of a joke. Hackers are typically clever and often self-serving. By definition, they fabricate tools for themselves which, like anything we have, can be used for good things and bad alike. Many of those tools come into popular use. It's difficult to know what, actually, the intention was behind the tool's creation.
(It wasn't created for cracking/phreaking, but it was created by phreaks. Woz and Jobs' first venture was selling blue boxes.)