Yabup reads and to a lesser extent, writes a pile of different file formats.
    It is pretty basic, because I haven't spent much time on it. You can zoom, pan, rotate etc. by pressing mouse buttons and dragging.

       Load / Save ASCII & Binary STL.
       Load / Save TRI format
       Load / Save OBJ format (just pulls out the polygons)
       Load / Save RAW format
       Load LWO Lightwave format (does colours)
       Load 3DS pulls out the triangles ok. (quite cool in fact (no colours))
       Load POV (pulls out triangle and smooth triangle declarations so it isn't very good)
       Load DXF (pulls out the 3dface declarations (no figures etc) so it isn't very good)
       Load NFF ok, ish
       Save VRML 1.0 (pretty basic)
       Load COB ok, ish (Caligari Truespace)
       Save OpenGL source code (can be handy)
       compiles under Linux (if you're lucky and willing to fiddle with the makefile)(requires Mesa and WxWindows2.0) or
       compiles under Windows MSVC++v6 (requires WxWindows2.0)

Download Yabup  HERE(exe) or HERE(zip) or Source Code. Released under the Gnu GPL.

STL Example Files 1, 2, 3. There are loads of 3DS and LWO files around on the web which are a whole lot cooler