Some Good Sites to Check for Malicious Content

I really like McAfee's http://trustedsource.org and BrightCloud located at http://www.brightcloud.com. I paste the ip that I think is bad or malicious into their reputation database. Then I can block accordingly in my firewall. To look for a particular ip address in your apache logs type the following.
grep {ip} /var/log/httpd/*

Typically your apache logs are all located in /var/log/httpd/

I also like this site. If your business does not need access to overseas or no one needs to connect to these netblocks you can use these in a block list as well.
http://www.countryipblocks.net/networking/top-10-global-spammers-1q-2010/