There may soon be three more letters in your employer's alphabet soup of benefit plans. Those letters are HSA, which stands for health savings account, and they can bring tax advantages at a time when you could use them the most.

Financial planners get all googly-eyed when talking about these accounts, though not because they're a way to save pretax money to pay current medical costs. Instead, they rave about them as a way to save money to use for future medical costs—after you've retired. The key to that appeal comes from three magic words: triple tax-free. 

The number of HSA accounts increased 16% year over year as of June 30, surpassing 21 million, according to a survey by consulting firm Devenir. Assets held in HSAs rose 23%, to $42.7 billion. As companies shift more health-care costs onto employees via plans with high deductibles, HSAs let workers put pretax dollars into savings accounts for medical expenses. They're similar to flexible spending plans (FSAs) but without the annual "use it or lose it" requirement—contributions can roll over year after year. Any earnings increase tax-free, and the money isn't taxed when used for qualified medical expenses. People who don't use direct deposit at their job can contribute to an HSA but must wait until tax time to get money back by claiming a deduction.