I use "open source" personally, but it depends if there is ambiguity.<p>"Open-source" is a compound modifier which are used to avoid ambiguity.<p>Example:
"Man-eating shark" could be read "man eating shark" without the hyphen, which can change the meaning to a man eating a shark instead of a shark eating a man.<p>Wikipedia switches constantly:
<a href="http://en.wikipedia.org/wiki/Open_source" rel="nofollow">http://en.wikipedia.org/wiki/Open_source</a><p>"open-source" occurs 138 times
"open source" occurs 119 times<p>I can't think of a scenario where "open source" leads to ambiguity. I would argue "open-source" removes any chance of ambiguity and is the better choice, but for aesthetic purposes I prefer "open source".
I usually use "open source" but I try to refer to the actual license to avoid ambiguity whenever possible. e.g: rather than say "This repo is open source" I'd rather say "This repo is MIT licensed".
I use Open Source and Free Software (capitalized) as well as free as in beer (lowercase).<p>No one else seems to be picking up on my typographical hacktivism though.