Long ago I used to view stuff with a Google viewer called Picasa because, back then, it was the only non-Sigma app that could open my .X3F raw files from my Sigma SD DSLR on my (Adobe-free, no ACR) computer.
Then it disappeared from the web, bemoaned by many at the time. Today I discovered that it still lives as stand-alone app, albeit somewhat tightly connected to one's Google Photo accounts - of which I have a few.
download: www.filepuma.com/download/picasa_3.9.141.259-10729/
notification 2016: support.google.com/picasa/answer/6383491
Don't know if it does Mac or Linux, sorry.