Even though many zero-percent-APR introductory offers have been getting longer lately, a low-interest credit card can help you prepare for an emergency expense or for a moment when you might not qualify to surf your balance interest-free.

PenFed Promise Visa Card

Although credit unions don't have to ramp up interest rates to please shareholders, very few of them have the reach to run their own credit card programs. Many credit union credit cards are just "white label" products operated by major banks through partnership agreements, offering the same features -- and similar interest rates to their better-known counterparts. As one of the largest and fastest growing credit unions in the United States, Pentagon Federal uses its size to break the pattern.

The PenFed Promise Visa® Card carries no fees, and its variable APR uses one of the market's lowest spreads against the Prime Rate. At the time of this writing, this card carried a variable purchase APR of just 9.99 percent. In addition, PenFed often runs balance-transfer deals on this card with no transfer fees and a fixed promotional rate (currently 4.99 percent) for the life of the transfer. With a strong track record of paying your bills on time and staying below your total credit limit, you could qualify for a card that can really help knock down your remaining credit card balances.

Barclaycard Ring MasterCard

Meanwhile, an upstart division within one of the world's biggest credit card issuers keeps running an experiment to see what their operation might look like if they ran it more like a credit union. The Barclaycard® Ring MasterCard® runs on a set of community principles shared on a cardholders-only website. Members help decide everything from service fees to charitable donation recipients. As a result, Barclaycard Ring now features a low, variable APR (currently holding at 8 percent) for purchases and balance transfers, no balance transfer fees, and relatively low cash advance and foreign transaction fees.

When Barclaycard Ring first launched, it seemed like a clever little test to help establish an independent brand name in the United States. After all, the Wilmington-based bank had offered private label retail and airline credit cards for years. However, as those licensing deals expire, the bank has started migrating more of its customers over to Ring. According to company bloggers "Jen" and "Jared," as many as 14,000 new cardmembers join Ring every month, suggesting that this low-interest credit card might stick around for a while.

Iberiabank Visa Classic

Credit cards don't all come from Delaware and South Dakota, though. Some of the market's cheapest credit cards originate in Louisiana at Iberiabank, a regional lender that started over 125 years ago as a building and loan office in Lafayette. Since then, Iberiabank has grown through a series of strategic mergers and by carefully vetting the customers to which it loans money.

With a strong credit report and the willingness to submit up to six months of your company pay stubs for review, you could qualify for an Iberiabank Visa® Classic card with a variable purchase APR that currently sits at 7.25 percent, 10.25 percent or 13.25 percent, depending on creditworthiness. You won't get too many bells and whistles on this card that's ideal for emergencies.

Simmons First Visa Platinum

Keeping interest rates extremely low often means giving up a few perks, like a customer service phone line that gets answered outside of business hours. (Don't worry, there's still a special, 24-hour hotline to report your card lost or stolen.) If you're willing to trade some urgency for personal attention to detail, the Simmons First Visa® Platinum also features an extraordinarily low variable standard purchase APR, currently 7.25 percent.

It could just be how banks prefer to do business in the South, but Simmons First (like Iberiabank) often asks first-time applicants for proof of income and employment stability far beyond what's listed at the major credit bureaus.

US Bank Visa® Platinum Card

Although Discover, Bank of America, Chase and Citi have been duking it out over the no-frills market with variable purchase APRs between 10 and 12 percent, US Bank managed to sneak onto the list with a Visa card that has a variable purchase APR as low as 9.99 percent (full range is currently 9.99-23.99 percent, based on creditworthiness when you open your account). While this card's 4 percent balance-transfer fee (or $5 minimum) trends slightly higher than its competitors, a 12-month zero-APR introductory period on purchases and balance transfers makes this a good choice for managing an unexpected, major purchase.

This card also features FlexControl, a collection of online credit card calculators and other budgeting resources designed to help cardholders pay down balances faster. It's another sign of a trend among credit card companies that want to reduce risk in their lending portfolios by attracting consumers who want to get out of debt.

These five cards may not come with lots of extras, but they will help insulate you against higher finance charges when the Prime Rate eventually rises.