A good article about when to use taxable accounts. Generally, it is best to use an employer sponsored tax-deferred account (e.g., an IRA or a 401k plan) when saving for retirement. Such accounts typically allow for pre-tax contributions, tax free growth of your money, and sometimes even include employer matching contributions. However, some folks may not have access to an employer sponsored plan and are limited to the $5,500 or $6,500 (for those 50 and over) one can contribute per year to an IRA. Although, saving approximately $6,000 per year is not a bad start, saving even more improves your likelihood of retiring earlier and living life a little more comfortably. Hence, the first instance of utilizing a taxable account is to save more for retirement. Such taxable accounts should take advantage of tax managed assets, i.e., securities that don't provide regular capital gains and dividend income but focus on stock price appreciation, index funds, or tax free bonds. Other instances ripe for utilizing a taxable account include:

You wish to retire before age 59 1/2; such taxable accounts would not be subject to the early withdrawal 10% penalty;
You are saving for a long term goal (a long term goal other than college - you can use a 529 for college goals); or
You may wish to actively trade or speculate with a small portion of your money.

A penny saved is a penny earned, even in taxalbe accounts!