Haiku is described as 'Open-source operating system that specifically targets personal computing. Inspired by the BeOS, Haiku is fast, simple to use, easy to learn and yet very powerful' and is a very popular operating system in the os & utilities category. There are more than 100 alternatives to Haiku for a variety of platforms, including Linux, Self-Hosted, BSD, Windows and Mac apps. The best Haiku alternative is Linux Mint, which is both free and Open Source. Other great apps like Haiku are Ubuntu, Debian, Fedora and Arch Linux.
If you look for alternative to Haiku, you've certainly used Ubuntu and it's either too heavyweight or too mainstream for you.