Windows, Linux, Mac: Hawkscope is a Java based free and open source tool that adds a little icon to your system taskbar, clicking on which opens a pop-up menu using which you can browse your hard disk contents and open them.
For the keyboard junkies, there is an option to set a key combination that'll open up this pop-up menu. You can set folders as your favorite ones for much faster quick access. You can also blacklist certain folders so that they are not included in the pop-up menu listing.
The application is quite extensible via plugins. Currently, there are plugins to search of Google, check Gmail and (obviously) a plugin to post to Twitter.