Download Open Freely now and you won't have a single problem installing or using it. The installation process is simple, and using the app itself is very pleasant - the interface is sleek and simple, the icons clear and interaction smooth. Open Freely is a pleasure to use.
In so far as supported files, Open Freely will open a huge variety of standard formats. It doesn't stretch to anything particularly unusual, but really covers all the basics, from Office Documents to Audio files and Windows system files to coding docs.
What makes Open Freely stand out is that it has some extra tools that will come in really handy. You'll be able to view details of all the files you open, as well as opening any of them in a hex editor, which hardcore users will appreciated. What's more, you can even open and edit Word documents, something you definitely don't see every day.
As far as free file viewers go, Open Freely is one of the best we've seen.