I've been searching around a little to find potential solutions for this and figured I'd share what I have so far in the hope that others can chime in on this subject.
Scenario: You have a widescreen monitor and you want to play your games in their original aspect ratio, but you don't like having black bars with nothing of interest on the sides of the game. Now you could use the bezel system that some people have set up for sure, but what if, like me you have a rotating monitor, the bezels won't work for that and they are also a lot of work to set up for each game.
My thought: Make your desktop background into a generic patterned bezel and then run games in a borderless, maximized window with a maintained aspect ratio in the center of the screen. This way the generic background bezel shows around the game edges where the game doesn't use the whole desktop. You could even make an animated video desktop and have that showing around the edges of the game.
So far I have seen a few people talking about solutions.
Here's an AutoHotkey script that can be configured to do the job...
http://gaming.stackexchange.com/questions/17306/how-to-force-maximized-fullscreen-mode-in-any-gameHere's a little app called 'Games Window Relocator'
http://gwr.orekaria.com/Another called Shiftwindow
http://grismar.net/shiftwindow/I haven't had time to try any of these methods yet, but I was just wondering if anyone else had managed to run Mame in a borderless window using the maximum screen height/width for the game's aspect ratio while still being able to see the desktop in the unused areas.