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There is one system
Not a multitude, you see
Think before you fork!

The Haiku OS thankfully is not like Gnu/Linux in that it is split into literally hundreds of conflicting and confusing distributions (see here), often which may not be well-designed. Rather, Haiku's simplicity and unified, minimalist structure makes it much different, and like BSD, only a few derivatives exist.

The project generally discourages choosing to make a distribution, unless one has "thought long and hard about doing so" (see this link). This is done in the hope that the Haiku OS will remain unified and not fragmented and virtually ruined as others have become. However, as the project is open source (mainly MIT-licensed), if one follows their guidelines, making a derivative is possible. 

For the curious, as of June 2015, the only derivatives (i.e. forks and distributions) known to be in existence on the Web are:

Coupbit - originally codenamed "Newleaf", announced on the Haiku forum in mid-2014
Status: Active

Status: Dormant (last release: 2011; original mirror down. Sorry. Hopefully, a link to a new mirror will be found for this link soon. Thanks to korli for the update.)

Status: Inactive (0.1.0 released in 2007)

Other

TiltOS - this is a set of packages meant to add X.org compatibility to Haiku.

ZevenOS - this is a Gnu/Linux distribution designed to look like BeOS.