For the people who can't wait for Google to release the filter results feature, do it yourself.
1. Go to http://www.google.ca
2. Save as... to hard drive.
3. Open in Notepad or any text editor.
4. Search for form tag.
5. replace with this:
form name="f" action="http://www.google.ca/search>
6. Search for maxLength=256 to find q input text field.
7. Add input maxlength="256" size="55" name="q" value=" -site:about.com ">
8. Save results and exit.
9. Double click google.htm web page.
10. Bookmark or set as home page.
This isn't the most elegant solution, but it does the trick to get rid a few of those annoying sites that always seem to get clicked on somehow and then you try to register and it doesn't work and then they take your email and sell it to a spam company or somebody in Nigeria and then you get some email from a long lost relative who has $20 million dollars in trust for you but needs $7,000 to ship the documents to you but you don't have $7,000 dollars so you wish you could have filtered the web site out in the first place.
Works for other stuff too, click the Advanced Search and play around with the parameters yourself.
Update: About.com is a very useful site that was an example and should not be filtered and in no way is associated with selling information or people from Nigeria, from what I can tell.
Update 2: Not all Nigerians have $20 million dollars of your money and need $7000 to ship it to you.