Mint, a site that allows users to track their bank, credit card, and investment accounts through a single free service, just announced that it signed up its 1 millionth user on Sunday.

The news is just the latest sign of the Mountain View, Calif. company’s rapid growth since launching in a crowded field of finance sites at the TechCrunch40 conference in September 2007. It looks like Mint has become a hit through its intense focus on usability — in other words, chief executive Aaron Patzer and his team realized that the most effective way to win people over was to make the site as easy-to-use as possible.

That focus may have come at the cost of some other features — Mint has placed a much lower emphasis on social tools, and Marc Hedlund, chief executive of competitor Wesabe, also likes to point out that Mint didn’t build its own online banking solution, but instead uses Yodlee, another competitor, as the underlying technology. Judging from Compete‘s numbers, those aren’t big disadvantages to most users, since Mint has quickly outpaced the competition. In fact, Mint’s self-reported growth has been stupendous enough that financial software company Quicken sent a skeptical letter pressuring Mint to release more proof of its growth.

(Update: I asked Mint for more details on what the 1 million users number includes. It doesn’t cover people who created accounts then deleted them. About 70 percent of those 1 million users have entered financial information and are receiving updates from Mint. But part of that 70 percent is probably like me, having entered information but gradually ignoring the updates and becoming too lazy to visit the site.)

It looks like Mint’s popularity has been noticed outside the tech world, too — Patzer says he was one of the 20 “young business leaders” who went to the White House earlier this month to offer their perspective on the economy. He’ll also be speaking at the Finance 2.0 panel at the South by Southwest conference in Austin today.

SmartyPig, a service that lets users save money for specific goals, and which encourages that saving through social networking tool, will also be appearing on the panel, and in conjunction is launching its OFX Solution, which allows users to sync their SmartyPig accounts with personal finance sites like Mint and Wesabe. The Des Moines, Iowa company’s emphasis on savings over debt certainly resonates in the current economic climate — assuming people are still making money that they can save — and the new integration was apparently the most-demanded feature among its users.