As of today (6 January 2009), Google has been ranked as the third worst "spam service ISP" by Spamhaus.org.
Besides not doing enough to stop dodgy drug companies redirecting unwary consumers through Google Docs, the company has been serving as a major host to those intent on committing advance-fee fraud (one example being the "Nigerian" or "419 letter"), mule fraud (in this case, setting up an email account as a fake "intermediary" or escrow service to dupe an unsuspecting punter), and excessive spamvertising through its Gmail and Blogger services.
No respectable ISP wants to be seen as pro-spam, but the fact is that some companies permit spammers and scammers to operate through their networks. Simple greed could be the motive, or it could be that some companies simply are not targetting their resources effectively to minimise the abuse of their networks.
Microsoft used to be a regular visitor to Spamhaus"s dubious top ten, but they seem to have sorted themselves out. According to Richard Cox of Spamhaus, "Microsoft got rid of the bad guys, and off they went to Google, which is now hosting a lot of the stuff that was on Microsoft"s domains."
Google, it seems, has a way to go.
Update (7 January 2009): Google has, impressively and apparently overnight, disappeared entirely from Spamhaus"s top ten.