One of the interesting arms races is between google and all the services that seek to increase page rank. (I found particularly fascinating on this page the comment from a seo ("search engine optimizer") that "Google created this market and now they try to destroy it".) Basically, lots of people are offering to adjust pagerank scores for web sites for money. They work in different ways, but one of the easiest is to have farms of link pages, and work hard to get other people to link to your link farm. Then, anyone who pays to get on one of your link pages gets a nice boost in pagerank.
It will be interesting to see how this plays out over the long term. Has anyone seen research that describes the relative strengths and weaknesses of the two positions in this arms race?