Open source alternative: http://en.wikipedia.org/wiki/J_(programming_language)
Although, like I said... most k programming is done in the context of applications built on top of kdb (their in-memory datastore), and I don't believe there's an (open-source) J equivalent.
Is J Open Source? Last time I looked it wasn't.
From the web site
"Jsoftware licenses source for all the Jsoftware binaries. There are several factors in determining the cost of source licenses. Primary is the source itself: J Engine (portable C); Windows GUI support (C++); and Unix GUI support (Java). Secondary is the scope of use: internal; distributed products; and competitive (to Jsoftware) products. And finally the platforms: Windows; Unix; PocketPC; etc. The price range is from $10,000 to $400,000. Regular updates to our current source levels are available for separate update fees."
I was able to get j7 to build on OpenBSD (with a bit of tweaking), FWIW. K/Q seem to be much more my style, but I haven't had any success there. To be honest, I'm more curious about how they're implemented, though - I may do my own eventually.