View64 can be used to view native C64 image files without firing up an emulator with the respective editor.
For realistic display it supports PAL/NTSC display with scanlines, shadow masks and other artifacts.
Pvnc is a portable VNC package. It can be built with extensions (e.g. open GL). It is tested with FC-6 and FC-8 and should work with other Linux distributions such as Ubuntu.
Doris is a Lua script driven OpenGL viewer with GUI widget extensions. Lua is a fast, powerful, portable scripting language. Lua bindings are provided to OpenGL, GLUT, GLUI (a GL widget library) and Luasocket (networking).