Rat Out Your Competitor
[January 2015 update below]
Jill Whalen, noted Boston SEO guru, is conducting an experiment to see if Google actually cares about how websites cheat to get to the top of their search results. Her guess is that they don’t, or they would have done something about it a long time ago.
You can participate in that experiment. If you have competitors who are beating you out in search engine results through spammy, deceptive, or unethical SEO practices,
go to the website for Rat Out Your Competitor. Let them know what your competitor is doing, and they’ll look into it and report it directly to Google.
[Thanks to Dean Voss for pointing out in the comments below that RatOutYourCompetitor.com is no longer what it once was. A better way currently is to report webspam directly to Google via their Webspam Reporting Tool.]