Google announced this week that they will be aggressively targeting spammers who hack legitimate sites and use them to download malware or promote traffic to low quality sites or those offering porn or sales of illegal goods. They have begun rolling out algorithm updates to identify and filter out these sites.
It is estimated that about 5% of queries will be impacted so you might notice something different the next time you search. Because they are only including the most relevant results there may be fewer of them.
Google’s official statement can be read on their Webmaster Central Blog.