One of the very positive (in my opinion) new directions that the web is evolving in is the social voting for content that takes place on the more proactive and better web sites. Two sites immediately spring to mind when I talk about this. They are Digg and del.icio.us. Sites that appear under the "most popular" category are usually justified in being there. Both of these web sites use a voting system by registered users to push popular content to the top of the list and both include spam fighting software to prevent malicious spammers from promoting self-interest and spam content.
One of the problems that comes with the web is spam web sites with duplicate or spam content. Google are doing a good job of eliminating spam content from their indexes by using humans to check sites.
There is another solution for Google, Digg, and del.icio.us that would involve social anti-voting. By allowing registered users to block certain web sites (and monitor which sites users blocked) would give them the power to see which sites the users didn't want to visit every again (via their web site) and form a social blacklist. This social blacklist could then be checked by the moderators and if justified be de-indexed from their sites.
Of course this does have some problems. What if a user blocks a site that is useful but does not interest them? The solution here would be to allow the blocking user to select a reason for the block. For example: "spam", "not interested", "not my native language", "other" etc.
What if a user registers lots of accounts to try and spam block a competitor? This should be obvious through IP address discovery or by a quick human check of the site.
What happens if you block a bunch of web sites now that you have no interest in but in a few months time that becomes an area of interest for you? Well Google already has a feature at the bottom of their search page which says something like: "We have excluded content which is similar... but if you want to re-run the search including that content then click here..." So, these web sites would include the same sort of line "16 results not shown because they are on your block list. Click here to show block list results."