Check out this free disk space gadget over at the Microsoft Gadget Gallery. The meter lets you know instantly how much free space you've got on your hard disk. As with any gadget you can add multiple instances of it to the Sidebar, which means you can monitor as many disks as you want.
Give your Vista apps a presence on the user's desktop by using Gadgets and Windows Sidebar in Vista. It gives your user a new path into your app, and can help with monitoring, dashboarding and alerting when it comes to important functionality.
Of course, this is Vista, so Gadgets are secure - they only run in Standard user mode so they can't get up to mischief on the user's machine.
Learn more about developing Sidebar gadgets at the Gadget Builder Depot.