One battle serves as the archetype for the rest: the fight over the VCR.
When Sony created the VCR, film studios were outraged. The studios had a DRM supplier they preferred, a company called Discovision that made non-recordable optical discs. Discovision was the only company authorized to play back movies in your living room. The only way to get a copyrighted work onto a VCR cassette was to record it off the TV, without permission.
In their lawsuit, the studios argued that Sony -- whose Betamax was the canary in this legal coal mine -- was breaking the law by unjustly endangering their revenue from Discovision royalties. Sure, the studios could just sell pre-recorded Betamax tapes, but Betamax was a read-write medium: they could be copied. Moreover, your personal library of Betamax recordings of the Sunday night movie would eat into the market for Discovision discs: why would anyone buy a pre-recorded video cassette when they could amass all the video they needed with a home recorder and a rabbit-ear TV antenna?
The Supreme Court threw out these arguments in a 1984 5-4 decision, the "Betamax Decision." This decision held that the VCR was legal because it was "capable of sustaining a substantially non-infringing use." That means that if you make a technology that your customers can use legally, you're not on the hook for the illegal stuff they do.
This principle guided the creation of virtually every piece of IT invented since: the Web, search engines, YouTube, Blogger, Skype, ICQ, AOL, MySpace... You name it, if it's possible to violate copyright with it, the Betamax decision made it legal.
Unfortunately, the Supremes shot the Betamax principle in the gut two years ago, with the Grokster decision. This decision says that a company can be found liable for its customers' bad acts if they can be shown to have "induced" copyright infringement. So, if your company advertises your product for an infringing use, or if it can be shown that you had infringement in mind at the design stage, you can be found liable for your customers' copying. The studios and record labels and broadcasters love this ruling, and they like to think that it's even broader than what the courts set out. For example, Viacom is suing Google for inducing copyright infringement by allowing YouTube users to flag some of their videos as private. Private videos can't be found by Viacom's copyright-enforcement bots, so Viacom says that privacy should be illegal, and that companies that give you the option of privacy should be sued for anything you do behind closed doors.