Researchers with Marshal's TRACE team have identified the top 6 botnets in terms of each's share of the world's total spam production:

Srizbi, 40 percent of spam
Rustock, 21 percent
Mega-D, 9 percent
Hacktool.spammer, 8 percent
Pushdo, 6 percent
Storm, 2 percent

Mega-D shut down briefly after landing on Marshal's radar but appears to be back in business.

"That spooked them and they shut down," says Michael Whitehurst, vice president of global support for Marshal."But this past weekend, Mega-D fired things up and started spamming again. Mega-D is known for its male enhancement spam, and interestingly enough, when it shut down, that spam kept going. The expectation was that the whole Express Herbals/enhancement spam would die out. But it didn't. What we saw was that volume shifted to other botnets. We saw four of the other major spam bots sending it - Pushdo, Hacktool.spammer, Rustock, and Srzbi. It makes you wonder about the relationship between all of them."

Read more of the report here.