Mikola Lysenko https://0fps.net
Graphics programming, geometry and algorithms.
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Steven Wittens http://acko.net
Not underrated in any sense of the word, but I have to include it in case there are people who haven't seen it yet. It's about math and graphics, but also about art and design. Amazingly educational. http://acko.net/blog/how-to-fold-a-julia-fractal/ is a good starting point for new readers.
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Mark Nottingham https://www.mnot.net/blog/
Networking and HTTP.
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Jeff Preshing http://preshing.com
Efficient concurrent data structures.
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Bruce Dawson https://randomascii.wordpress.com
Software efficiency, performance optimization, fractals and more.
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Yann Collet http://fastcompression.blogspot.com
Modern compression algorithms.
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Gil Tene http://stuff-gil-says.blogspot.com
Also definitely not underrated, at least not in the Java world. But any engineer who cares about latency and honest benchmarks will benefit. Low latency techniques, GC performance, lock-free programming.
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Tony Finch http://fanf.livejournal.com
C and UNIX programming, and more.
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Andy Wingo https://wingolog.org
Scheme, Compilers, JavaScript, and much more.
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Aleksey Shipilёv http://shipilev.net
JDK internals, low-level optimization.