I spent (wasted) 2 hours yesterday going through incoming links shown in Google Webmaster Tools. Of the roughly 165 sites linking to one of our sites, I disavowed 74 of them. I checked every site if I didn't recognize the domain name. The sites I disavowed were nothing more than spam sites. We never asked for these links. I don't know why they linked to us. What's more, I found incoming links from some of those same spam sites to another site of ours. I can't figure out why they would do that unless it was to harm our rankings.
On the site I worked on yesterday, Google keeps killing its traffic. At one point when it ranked #1 in Google for about 2 weeks back in 2008, it was getting around 2,000 visitors from Google per day. But Google keeps turning off the traffic. Before yesterday, the last time I checked our Google Analytics we were getting about 25 visitors per day from Google, compared to about 200 - 300 from Yahoo/Bing. The time I checked before that we were averaging about 50 per day. Yesterday, I saw that we got 22 visitors from Google in the prior 30 days! Now Google is sending less than 1 visitor per day to a site that it used to refer 2,000 visitors.
I don't blackhat. Nothing bad or tricky. Yahoo and Bing have held the traffic consistent for years. Google keeps killing it, and sending me emails trying to get me to pay for advertising. Is there a connection? I noticed on another site that traffic from Google dropped around Oct. 23rd, which is just days after Google made a disappointing earnings announcement.
Disavowing links isn't difficult. It does take time, which is a complete waste and should be unnecessary. You can find instructions here:
Instructions For Disavowing Links From Google Webmaster Central