Unique to DragonFly, which is a shame[1], since it seems to have implemented everything one could possibly want, including replication.

[1] In the sense it's a great filesystem that could be more widely used.

Currently, I don't think there's a fully working HAMMER implementation for Linux - there may be a read only one floating around though, IIRC

Maybe with fuse. No one has ported it too FreeBSD either which is to bad because having choices is a good thing™

Genuinely curious... why is there a "™" at the end of "good thing"?

"Good Thing" is often capitalized, or as above italicized, for emphasis in hacker circles. The idea is that it's established jargon [1] suggesting more specific meaning than a casual goodness remark (whether that's true or not, I'll let you decide). The TM is another such intensifier, with the informal meaning of "distinctive or characteristic", rather than literally trade-marked.

[1] http://www.catb.org/jargon/html/G/Good-Thing.html

i.e. Meme-marked.

It's very much the inverse of making a statement like /replacing all domain specific languages with bike shed driven development/ would be a bad thing™

It's a slang.