I've discovered that whatever language you want is probably available free. More of a constraint - and that not a severe one - will be the intended platform of use, either for the development environment or for cross-platform executable generation.
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The one thing that the Open-Source scene does really well is development tools. Because they're programmers - by definition - they use their own development tools, 'eating their own dogfood' as the saying goes.
The upshot of this is that there are many, many free programming environments available for download off the interwebs. The quality of these tools ranges up to best of breed. In fact, the market for commercial programming languages is pretty much dominated by platform vendors with a vested interest in having their platform supported by good development tooling. Independent companies selling commercial development tools are getting pushed further into the niches of enterprise and embedded systems development.
If you look at major commercial vendors of C compilers (for example), you will see Intel, Microsoft, ARM, a handful of niche/legacy players and hardware vendors such as IBM or Oracle (nee Sun) who sell compilers targeted at their hardware platforms. Everything else is variants of clang or gcc, which are both free. Most of the commercial players also offer freebie versions of their compilers.
Some systems (python, for example) are only available as OSS. There are no commercial Python tool chains that are not based on one of the open-source variants (CPython, IronPython, Pypy, etc.)
Most of the free systems will also have ports to all mainstream platforms - and often many more obscure ones. You can run Python, for example, on RiscOS, Z/OS, OS/400, OS/2, VMS and quite a range of other platforms as well as mainstream ones like Windows or Linux.