Retirement savers who have contributed the maximum to their 401(k) plans don't have to look far for the next big growth opportunity for nest eggs: It's right in their health-care plans.

Say hello to the health savings account, which works in tandem with high-deductible health insurance.

HSAs offer a triple-tax benefit: Assets in them grow free of taxes. Savers can contribute to them on a pretax or tax-deductible basis. Finally, account holders can tap the assets free of taxes, provided the money goes toward qualified medical expenses.

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Used wisely, HSAs are a new tool in retirement planning — and financial advisors can help with that.

"If you're expected to spend $275,000 on health care in retirement, where can you get the most bang for your buck?" said Tom Vipond, sales consultant for TD Ameritrade Self Directed Plan Services.

Here's where you can find the best opportunities for health savings accounts.