Removing letterboxing or pillarboxing from video clips typically requires expensive video editing equipment. This tutorial (and screencast) describes how to do this with the inexpensive and ubiquitous quicktime.
A screencast demonstrating this technique can be found at the bottom of this tutorial.
Covering unused screen with black is called windowboxing and is frequently used to preserve aspect ratios when changing video formats.
Letterboxing places blackbars on the top and bottom of the screen and is used to display widescreen format on a standard sized screen. Pillarboxing places black bars on the sides of the screen and is used to display standard sized format on widescreens. While these techniques are good compromises on television screens, often on a computer system these black bars are just annoying.
Quicktime Pro is required for these features.
1. In Quicktime Pro load up your video clip. Find a frame that is filled with a color except for the black windowboxing.
2. From the menu select Edit and then Copy to put the current frame into the clipboard.
3. Open up any image editor. Create a new document and paste in the image containing your frame from the clipboard.
4. Select the letterbox or pillarbox (or both) and fill those sections with white. Select the rest of the image and fill that middle section with black.
5. Save this image as a new gif with any name.
6. In quicktime go to the Window menu and select Show Movie Properties. Select the video track and the Mask box will appear. Drag your gif into the Mask box. Instantly the video clip on the screen will be clipped of the windowboxing.
7. In quicktime select the File menu and then Export for whatever settings you wish.