Mac: Broom is a neat little app that sits in your menu bar and monitors your hard disk, letting you know whenever it gets too full. If that time ever comes, Broom will alert you and then suggest files and folders you can throw away to help keep your disk space from running out.
By default, that threshold is 80 per cent full, but you can set it to whatever you want. When you disk passes that mark, Broom's menubar icon will turn red and you'll receive a cute alert telling you it's time to clean up. You can dismiss the alert or ask Broom to offer suggestions of folders and files you can delete. If you want to delete any of them, just click the magnifying glass next to its name and Broom will display the file/folder so you can move it to the trash.
Aside from being generally useful, with OS X Lion's reduced reporting of available disk space (by default) it's nice to have a handy alert available in your menu bar as well. While you could do what Broom does manually, the convenience makes it a nice time saver and it's always good to be warned of dwindling disk space before it's too late.
Broom ($1.99) [Mac App Store]