FileBrowser allows browsing directories and files inside and outside of the current project and thus makes it easier to open files with IDEA that are not part of the current project.
I use IDEA not only for Java projects but also as a general editor for HTML, CSS and XML files. There's no convenient way to open files outside of an IDEA project in IDEA.
This is where FileBrowser comes in. There's also a small favorite management system included. So there's no need to browse the whole way to often used folders. You can also drag and drop files on FileBrowser or drag and drop them from FileBrowser on another application like Firefox, Internet Explorer etc.
I've only tested this with Windows XP, but it should work with other OSs since it only uses standard APIs.
Please visit http://filebrowser.beeger.net/ for a whole walkthrough.
This plugin requires at least IDEA 7.0 M1a.