A Symantec white paper entitled Zeus: King of the Bots [PDF file] provides an in-depth analysis of the Zeus botnet package.

"Zeus provides a ready-to-deploy package for hackers to distribute their own botnet," write authors Nicolas Falliere and Eric Chien. "The botnet is easily purchased and also freely traded online and continues to be updated to provide new features and functionality. The ease-of-use of Zeus means the Zeus bot is used widely and is highly prevalent, allowing the most novice hackers to easily steal online banking credentials and other online credentials for financial gain."

Click here [PDF file] to read the Symantec white paper.