Mplayerosx

I have implemented this feature. To activate this functionality, simply open PlayerWindow.m and paste the following text inside the @synthesize block. Easy.

- (NSSize)windowWillResize:(NSWindow *)sender toSize:(NSSize)proposedFrameSize
{
    if([playerController videoOpenGLView].video_aspect <= 0)
        return NSMakeSize(proposedFrameSize.width, proposedFrameSize.height);

    CGEventRef event = CGEventCreate(NULL);

    if(CGEventGetFlags(event) & kCGEventFlagMaskShift) {
      CFRelease(event);

        NSRect frameRect = [sender frame];
        NSRect contentRect = [sender contentRectForFrameRect:[sender frame]];
        NSSize minSize = [sender contentMinSize];
        float heightOffset = frameRect.size.height - contentRect.size.height + minSize.height;

        float aspect = [playerController videoOpenGLView].video_aspect;
        float pWidth = proposedFrameSize.width;
        float pHeight = proposedFrameSize.height;
        float snapWidth = pWidth;
        float snapHeight = pHeight;

        if(pWidth-frameRect.size.width <= pHeight-frameRect.size.height) {
            snapHeight = (pWidth / aspect) + heightOffset;
            if (snapHeight < heightOffset) {
                snapHeight = heightOffset;
                snapWidth = minSize.width;
            }
        } else {
            snapWidth = ((pHeight - heightOffset) * aspect);
            if (snapWidth < minSize.width) {
                snapWidth = minSize.width;
                snapHeight = (minSize.width / aspect) + heightOffset;
            }
        }
        return(NSMakeSize(snapWidth, snapHeight));
    }

    CFRelease(event);

    return NSMakeSize(proposedFrameSize.width, proposedFrameSize.height);
}

Hold the shift key to keep the window at the correct aspect ratio. Release shift to resize the window in freeform.

I chose to keep freeform resizing as the default behavior for two reasons: Firstly, using the shift key to scale an object with a fixed ratio (such as making a circle with an oval tool) is a standard and familiar way to handle it, and secondly, for a personal reason. I use a program called Cinch, which allows me to drag a window to the side or top of the screen where it will become docked and resized just like windows 7's 'aero snap'. This half-screen docking works well with freeform resizing, but looks bad with the snapping behavior. I use this docking feature more often than I manually resize a window, so I would rather not hold shift key to get freeform resizing.

Update: Now uses [[playerController videoOpenGLView] video_aspect] (which required adding an @property accessor to the VideoOpenGLView class) so the chosen aspect ratio is used, rather than always using the original.