Arthur Levitt Jr., Securities & Exchange Commission

It's hard to name a big U.S. company that was not affected by Levitt in 2000. From restrictions on auditors acting as consultants to a ban on selective leaks of market-moving information, Levitt, 69, was everywhere. But come February, the Chairman steps down.

Charles Jeffress, Occupational Safety & Health Administration

Jeffress issued controversial ergonomic standards to prevent workplace injuries caused by repetitive motions. Jeffress, 52, estimates the regs will prevent some 460,000 injuries a year, saving corporations an annual $9 billion. But critics say complying could cost 10 times that.

Joel Klein, Former Assistant Attorney General

Klein, 54, who resigned his post in October, put the bite back in antitrust enforcement. He won more than $1 billion in fines and settlements. He sued AMR's American Airlines (AMR), halted MCI WorldCom's (WCOM) merger with Sprint (FON), and, of course, won a court order to break up Microsoft (MSFT).