I have a neat little slide viewer/projector, a Hannimex Felitor DB200 which is both a projector and a viewer. It is essentially a fairly compact slide projector which can project a reasonably big image with the included 50mm lens, but you can also use it as a viewer by fitting the 'hood' screen in position on top of it over the lens which reflects it off a mirror onto the rear of a 20cm x 20cm matt screen which is angled at 45 degrees for easy viewing, it also allows 126 and 127 square slides to be viewed as well.
It gives a quite nice image on the screen with little to no distortion or fringing and the mirror is keystone corrected for when its reflected onto the screen so you do lose a tiny amount off the sides. Its also useful as you can view the slides in daylight as well using the screen.
It cost me £20 on ebay last year when I was looking for a slide viewer. You do occasionally see them on ebay, but you can also get the DB120 version which is even smaller by only having a 12cm square screen. I'll post some pictures of mine later.