Mint, along with Wesabe, is a new breed of site that tries to attack personal finance via 'Web 2.0' technologies.  Mint's concept is that you can have the activities of your various bank and credit card accounts
aggregated in one place to make it easier to track your cash flow, purchasing, etc.  They will also alert you to unusual spending in certain categories based on your history and suggest deals or discounts where they think you could save money.  They don't store your credentials on their servers so they claim that it is secure.

The trouble is, it tends to be a poor labeler of my transactions.  For instance, I recently got an alert saying that I had unusual activity on my "internet" purchases.  Turns out it was counting transactions for Eurostar and Brussels Airlines in them, which should be categorized as Travel.  Giving them the benefit of the doubt, I'm guessing they would do better for US transactions since they'd have a critical mass of users helping categorize these.  But that doesn't do me much good.

They did recently announce a beta program to enable users to view brokerage activity as well, which might make it more useful.  I'm not quite ready to cancel my account but so far Mint is not very useful.