What do you do when you run across websites that appear to be in violation of a search engine's guidelines?  Do you smack-talk the website to yourself?  Do you contact the webmaster?  Do you ignore it?

Google would like you to let them know about it.

The Official Google Webmaster Central Blog has published a detailed article on the "how" of reporting spammy web results:

So, the next time you can't help thinking that the ranking of a search result was not earned by virtue of its content and legitimate SEO, then it is the perfect moment for a spam report.

There are two ways to file a spam report.  An authenticated form is available through Google Webmaster Tools.  There is also an unauthenticated version of the form available.  What's the difference?  In a nutshell:  the probability of response.

If you file a spam report through the authenticated form, Google says it will always investigate.  To use that form you must register at Webmaster Central.  If you use the unauthenticated form Google will evaluate the potential threat of the report and decide whether or not to look into it.

I hate spammy results as much as the next guy.  But Google isn't the only search engine that's affected by spam sites.  Below are links and instructions for reporting spammy websites to Google, Yahoo, MSN/Live.com, and Ask.com:

Google

Authenticated Version:
     - Use Webmaster Tools

Unauthenticated Version:
     - Use this form

Yahoo

Use this form

MSN/Live.com

Use the General Feedback form:

• Select "Found Spam" in the Type of Feedback drop-down
• Enter the URL of the spam site in the text field at the bottom of the form

Ask.com

Use the Customer Service form:

• There's no specific spam reporting option (If anyone knows for sure, please let me know).
• I would assume you'd use "Questions about Search Results" or "Comments or Suggestions"