An early version of Gnash for Haiku has just been released for testing.
The developer Adrian Panasiuk has done an amazing job on getting Gnash working on Haiku, including porting a handful of libraries. To be able to enjoy it, the following is required: You need to have OpenSSL optional package installed, a Haiku-hybrid or Haiku-gcc4 build and BeZilla-gcc4.
The developer is still working on the port, and would like to get bug reports. And please have in mind, that both on Haiku and Linux, Gnash fails to load a vast amount of youtube videos.
Two screenshots of Gnash in action is available on haiku-os.it.