401(k)

Entrepreneurs have responded enthusiastically: Solo 401(k) assets now stand at $2.5 billion, and could hit $90 billion by 2012. It's easy to see why the plans are becoming so popular. As with ordinary 401(k)s, as well as SEP-IRAs designed for the self-employed, individual 401(k)s allow you to invest a portion of your income on a pretax basis—and your contributions grow tax-deferred.

John Fuchs was checking his 401(k) account online one afternoon when he saw something that seemed amiss.

Contribute 10 percent of your salary each month to your 401(k), and you're good. Start at age 25 making $50,000 a year, and earn a 3 percent annual raise until retirement.