Wikipedia has more on K and KDB+: http://en.wikipedia.org/wiki/K_(programming_language)
I'm still trying to figure out the APL language family. I'd love to implement one, eventually, but it's a ways off. (I've done a toy interpreter already, but that was just to make sense of some basics.)
http://www.youtube.com/watch?v=a9xAKttWgP4
Terse, cryptic, powerful. It's like doing magic tricks with code. (Not that that's a recipe for maintainable software, but that's an entirely different debate.)
There's also (meant to mention this before) A+, which is GPL, I believe:
I've read most of _J for C Programmers_, and, this week, I also got _APL: An Interactive Approach_. The mindset is starting to make a lot of sense. (I've also looked at A+, but I'd rather stick to the ASCII variants.)
$mode ascii
Having read more about APL, though, I'm feeling less inclined to write off the extended character setup. Still, it's an issue one needs to sort out upfront. I guess it's the APL equivalent of bitching about parenthesis, when either is a shallow detail compared to what's really novel about the languages.
Many languages seem to have shibboleth complaints, to show when people haven't made it past the surface - Lua indexes from 1, Lisp has the parens, Python has the whitespace, etc. Maybe there's a community value in that, I don't know.
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.