The more incoming links there are, the more a website can be visible on Google. Since this means links are valuable, a company launched* a marketplace where one can buy "quality" hyperlinks - where quality depends on the age of the website, its PageRank of course, how Alexa ranks it, etc.
I have read somewhere that Google is able to discover (through its TrustRank system) whether there has been an exchange of links between websites. Here, customers apparently must keep their agreement confidential, to prevent search tools from noticing the trick.
[EDIT, April 16, 2007] It seems that Google wants to fight paid links, which may be deemed spam
* apparently 3 years ago, although I discover it today