Obviously there are much more noteworthy open source software developers in the world than is represented in this list, but one of the criteria for placing people to this list here is that hey are at least somewhat available as freelancers or have otherwise caught my eye due to some, not always best achievement, but some interesting, peculiar, nonstandard way.
Another criteria is that the people in this list must be somewhat less known or at least not that famous among the majority of planet Earth software developers. For example, creators of many widely used programming languages(Java, C++, C, Python, Ruby, PHP) and creators of some popular applications or libraries are generally considered to be too famous for this list.
The reason, why many fine developers and gurus disqualify to be at that list is that they do not have a web page that is exhaustive or otherwise interesting enough. That's often the sole reason, why many awesome university professors disqualify to be at that list. I know that it's harsh, but this is a web page and if a person does not work on its web presence then, well, the person is ABSENT from the WEB SPACE and probably also from the academic culture of the future, specially in a global economy.
Robert van Loenhout, alias Calitha, Netherlands
Dean McNamee, alias deanm, Germany, pre3d.
René Karl Müller, http://renekmueller.com/, Germany, The Labs.
Andrew Gallant, burntsushi.net, USA.Massachusetts.
Alan Skorkin, skorks.com.
Martin Hinner, Czech Republic, martin.hinner.info.
Fabrice Bellart, France, bellard.org.
Anselm R. Garbe, Germany, garbe.us, suckless.org.
Jonathan Worthington, England, Sweden, Perl6.
Dwayne Richard Hipp, USA, SQLite, Fossil.
Niklaus Wirth, Switzerland, Oberon, Pascal.
Primiano Tucci, an Italian living at various places.
Low level algorithms with portable GUI-s.
Norman Hardy, an old-school American software guru,
who has worked on a veriety of projects, mostly
low level things, operating systems, etc.
Larry Doolittle, an American academic,
who is an author of many speed-optimized
Ralf S. Engelschall, a German, who
seems to have done a lot of really good work,
despite the fact that he exaggerates a lot
by claiming that he was able to do
proper software development at age 19,
at which he was probably roughly as good as any
other fine computer enthusiast.
Clifford Wolf, an Austrian, who
created probably the very first
fully open source HDL-tool-chain by
iCE40 FPGA configuration binaries.
Dmitry Grinberg, an American-Russian, who has some theoretically remarkable embedded projects.
Peter Thorson, an American freelancer, who has written WebSocket++, a C++ WebSockets library.
Joseph Chet Redmon, an American, who
seems to have both,
techical wits and general smarts.
Michael Ringgaard is a Dane, who
has created his own POSIX compliant
operating system, Sanos
R. Kent James is an American, who
has worked extensively on the
Mozilla Thunderbird project and
seems to be quite an adventurer.
Thomas Bushnell, formerly known as Michael Bushnell (born 1967),
is an American, who is/was one of the
main developers of the GNU Hurd operating system.
Bradford J. Rodriguez, is a freelancer,
who works on Forth and computer architecture.
Some Estonian gurus:
Ain Isotamm, an old-school Estonian software guru, but
unfortunately only Estonians can get a "taste" of him,
because he is very local, operates only in Tartu and
given his age and hopeless amount of smoking
only time will tell, how long he will
be able to work on anything at all.
I attended his lectures somewhere in 2003 or 2002
and in 2016 I am amazed that he is still in
any shape at all to actually pull off any academic work.
He is/was an awesome professor, but You'd better
attend his lectures while You can (and
hope that You get Your mark from him before his funerals).
He's not just smart, hardworking and kind-hearted,
but something more and his "flaws" just
make him more human. :-)
Jüri Kiho, an old-school Estonian software guru, who's
main contributions seem to be the development
of Computer Science terms in Estonian and
university level study materials in Estonian language.
Practically, he's one of the main guys, who
made sure that the Estonian language is not some
language of apes that can not be used for
talking about modern Computer Science topics.
As mentioned earlier, many Estonian
software development related people
disqualify to be listed here, because
their home pages are not exhaustive enough or
the home pages do not compensate the lack of material
with something otherwise interesting.