SEOMoz, one of the major sites for us SEO types, has a wonderful page where they seek to track major changes by Google.

Google Algorithm Change History

It's pretty instructive from a couple of angles. For one, you can see the recurring theme of overcoming the tricks used to make low-quality sites rank high. For another you can see the increase in sophistication over the years in what Google is doing.

Finally, back in late 2009 when Caffeine was rolled out, I predicted that Google would be making major changes at a much faster pace. And sure enough you can see that in the change from typically two major changes per year, to six listed in 2010 and nine already so far in 2011.