Omnispeak is a re-implmentation of the Keen:Galaxy engine which aims for almost perfect, bug-for-bug compatibility with the original keen 5 engine in modern-ish c99, based on several disassembly/decompilation efforts. It aims to be as similar as possible in design to the original Keen code, which seems to have been finally found after many years of hiding.
You can download binaries for linux here or for windows here (or here for a 64-bit version. A version for Mac OS X Mavericks (10.9) is here thanks to the amazing Ethan Lee. You can also check out the source code on github: https://github.com/sulix/omnispeak. You'll need a copy of Commander Keen 5 to play it. You can find it on Steam.
If you're interested, don't forget to check out Keen: Modding, a community of people who've been modifying Commander Keen for years; NetKeen, a multiplayer Commander Keen game whose source code has been invaluable in the creation of Omnispeak; and Commander Genius, a much more complete reimplementation, with more of a focus on playability rather than accuracy to the old engine design.
Extra special thanks go to NY00123 and lemm, who've put a lot of development work into making Omnispeak awesome.
More thanks go to adurdin, John Hodge and the entire Commander Keen fan community, who are keeping Keen alive. And, of course, none of this would be possible without Keen's original creators, thank you!