An improved version of that image mirroring program I made awhile back. This one allows you to select the line of symmetry, rather than always having it be at the center of the image. Left-clicking sets the line of symmetry. Right-clicking changes the location of the zoom-box and the left/right keys nudge the line of symmetry one pixel to the left or right, if you want more precision.
It gets kinda slow if you try using it on larger images, so smaller images will yield quicker results.
A java applet and Processing source code can be found here.