Soxy resolves a common problem when there are multiple versions of applications like Adobe InDesign, QuarkXPress, Adobe Illustrator installed on the same computer: double-clicking a file's icon will often cause the wrong application version to launch.
As a result, you might unwittingly re-save a document into a more recent file format, which could be a bad thing. Maybe it's a customer file, and the customer might not be able to open the updated file when you e-mail it back to her.
To work around this problem on Mac OS X, we've created Lightning Brain Soxy - with Soxy installed, there is no second guessing, and a double-click in the Finder will open the correct application.