SkyOS was quite prominent in OSNews between 2003 and 2008, it was coded almost entirely by the Austrian Robert Szeleney in his spare time. He achieved a relatively usable state at the desk, especially considering it was a one-man project - it was a hobby project for Szeleney, and as such.
SkyOS originally started as a simple boot manager, but eventually expanded into a full operating system. In the first few days, until version 4.x, SkyOS was available for free, but it was not open source. After that, Szeleney began charging a small fee for accessing the beta version of SkyOS 5, which some considered controversial.
The launch cycle of SkyOS 5.0 began with its first alpha version in December 2003, and ended with the release of beta 6947 in August 2008, a final version that never materialized.
The project seemed to fade away, but Szeleney experimented with the use of third-party cores, such as a BSD kernel or a Linux kernel. That was the last we heard from him, and earlier this year, the website was removed.