For thumbnails, I primarily use XnView (www.xnview.com, wonderful freebie). I also have:

- ST Thumbnails Explorer (not free), which is ok.

- Dimitry's X2 viewer for xar, cmx and ai files, although I haven't been using it as much since both XnView and ST Thumbnails can display limited thumbnails for xar files. (Still comes in handy. Thanks again, Dimitry. )

Plus, my file manager of choice (Explorer Plus, no longer in development) has a built-in viewer.