If a company manages a retirement plan for its employees, it has to file some paperwork (surprise!) with the U.S. Department of Labor and the IRS as well. The plans are covered by federal law, chiefly the Employee Retirement Income Security Act (ERISA). The plan filings, generally known as Form 5500's (although there are actually a whole series of forms involved), cover retirement plans such as 401k plans, pension plans, defined benefit plans, defined contribution plans -- the whole ball of wax.
Form 5500's are public documents meaning you can look them up on the Internet. You might be surprised at how much financial detail you can find about a company (and sometimes, about individual employees) through their Form 5500 filings.
Take a look at the Department of Labor's Form 5500 Filing Search as well as DOL's Pension Plan Actuarial Information Search. Also run the same searches at BrightScope, as this site often has retirement plans listed that, for some reason, doesn't show up at DOL.
Need more information? Newspaper archives are one of the best online research tools. If you're searching for information on people, try Intelius.