With the recent protests against SOPA and PIPA, the prosecution of Megaupload's staff, and the subsequent Anonymous attacks on the websites of various copyright enforcers (e.g. DoJ, RIAA, MPAA), it is time to distill this issue to its essence: copyright is slavery.
This statement may seem a bit extreme, since our own experience demonstrates that copyright is compatible with basic political and economic freedoms (more or less). Still, I stand by this blunt statement; copyright may be just a tiny slice of slavery, but it is still slavery.
Copyright is slavery because it empowers one person to prohibit another person from engaging in a fundamental human activity -- the sharing of songs and stories. The owners of copyright will argue that these songs and stories would not exist except for the efforts of the copyright owner (and perhaps the copyright system itself), and therefore the copyright owner does not deprive anyone of anything. This may be true in the narrow sense -- that a particular song or story would be unavailable -- but it ignores how these songs and stories interact with the human brain and are assembled into "a personal cultural catalog", as I'll call it. It is this catalog that is unjustly privatized by copyright, ultimately giving copyright owners a sort of ownership over other persons. Furthermore, the commercial value of copyright is largely derived from how humans relate to each other by sharing stories and songs, so copyright results in the privatization of our social lives by third parties and the centralization of cultural control.
Supporters of copyright dismiss the idea that our personal cultural catalogs are in any way relevant to the legitimacy of their copyrights. They point out that the assembly of our catalogs are our business, and that they did not force us to listen to their songs, nor did they force us to obsess over the characters of their stories. In some sense (they claim), copyright can be viewed as a contract between the producers and consumers of culture, where access to the story or song is only granted if the consumer agrees to respect the copyright of the producer; given this contract, if the consumer decides to weave the story or song into their own life, that is their own decision taken as a free person. Leaving aside the issue of whether slavery contracts are legitimate, this contract theory of copyright does not hold water. The main reason is that no such contract exists. Without an explicit contract (agreed to before any exchange takes place), a person cannot be expected to understand the implications of the restrictions that are being placed on him. Furthermore, there are only some situations in which such a contract could even be feasible -- such as when purchasing a book or entering a movie theater; the idea of a contract is absurd when copyrighted content is broadcast to our radios or televisions. Finally, the most important parts of our cultural catalogs are collected when we are children, and children cannot be expected to enter into contracts that will restrict their actions for their entire lives.
So that's my argument that copyright is a form of slavery. A quick Google search did not reveal any other arguments for this position. Still, if you want a more elaborate legalistic examination of the issue, I refer you to Stephan Kinsella.
Let's wrap up with a song (Copyright Slavery, by Der Plan [German with some English phrases])