According to the latest MessageLabs Intelligence Report, botnets are responsible for over 80% of all spam. Here's a snapshot of where the top 5 most notorious botnets currently stand:

Cutwail

The largest and most powerful botnet is responsible for 45% of all spam. With between 1.5 and 2 million active bots, Cutwail was perhaps the largest botnet in history at its peak. It experienced several hours of downtime on the morning of June 5, 2009, following the shutdown of California-based ISP Pricewert by the FTC and is now at 1/3 of its original capacity.

Mega-D

The top botnet at the start of the year, it has been on a steady decline and is now responsible for only 9.3% of spam. However, it's still one of the hardest working botnets in terms of spam per bot per minute.

Xarvester

One of the major botnets of the year, it has drastically reduced in size and output, sending out 0.2% of spam.

Donbot

Atop 5 botnet in size and output, it's been less active in recent months sending out 3.2% of spam

Grum and Rustock

These newer (but very large) botnets are a wildcard. Combined, they're responsible for over 10% of spam, but their activity is hard to predict: both send spam in bursts, with Rustock often going through periods of zero activity.

Source: June 2009 MessageLabs Intelligence Report.