According to a report by Spamhaus, 80% of the spam that users receive are generated by a group of hardcore spammers from North America and Europe. This listing is ranked by those that are the greatest risk to computer users, as they not only send out spam, but other harmful Internet threats such as malware, botnets, and pharming scams.
The list below is from the recent entries in the Spamhaus' register of known spam operations (ROKSO) as of January 2011

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Rank 1: Canadian Pharmacy
This gang uses botnets to send thousands of millions of spam per day with the help of its affiliates all around the world.
Rank 2: Rove Digital
Rove Digital is the name of a group that operates in the country of Estonia and are responsible for a number of botnets, pharming, malware, and other Internet dangers that face computer users
Rank 3: Alex Blood / Alexander Mosh / AlekseyB / Alex Polyakov
Massive botnet, child-porn spam ring, pharmacy and mortgage scam that originates in the Ukraine. Have been tied with fellow spammers Leo Kuvayev and Yambo Financials.
Rank 4: Vincent Chan / yoric.net
This Chinese spammer along with his brother runs several campaigns related to pharmacy, mortgages, ink cartridges, toner cartridges, watches and other OEM warez.
Rank 5: Peter Severa / Peter Levashov
A partner of Alan Ralsky and other spam gangs.