If you are nostalgic for BeOS, then the elevator pitch is, “It’s Be, only on modern hardware and more software support.”

If you are unfamiliar with BeOS, the pitch is: “Imagine an extremely lightweight desktop is with all of the things you would expect in a modern environment with none of the legacy. In an alternative universe, BeOS would have become OSX.”

There are so many things that Be did right from the very beginning that other OSs have adopted, but never as cleanly as Be did it.

For example, its file system. Most people don’t really notice or care about the file system, they all have directorys and hold files, maybe with permissions. BeFS does that as well, of course, but so much more. The entire file system acts as a database, so you can easily perform fast queries on it. You can also create virtual directories that are the result of those queries.

You want a “folder” that contains every markdown file created after 2020 between 20 and 1000kb in size? Bam, instantly done and live updated whenever something accesses it. The files aren’t actually copied there, just appear there to normal tools, almost like soft links.

BeFS also supports a resource fork system that it calls attributes. These can also be queried using the same database like tools as the rest of the system. File typing is done this way, every file gets a MIME type attribute and there is a daemon that sniffs them when a new file is downloaded or copied over.

Even more, this allows some crazy things like plain text files that have font, color and other formatting elements because all that is stored as an attribute.

Or their contact information app, which stores every person as a zero length file with details as attributes. You can create a virtual folder of all your contacts that meet a certain criteria and have other applications use that folder for whatever.

Or the email app which stores each email as a file, and adds the basic metadata like to, from, subject, read, etc as attributes. Then you can have different virtual folders based on those. This also means that the basic file system browser is the default way to view email, because it supports all the attribute viewing, queries and such you would need. Or you can do it all from the command line using either basic cli tools or some slightly specialized ones.

Combining attributes and virtual directories makes for a fantastic media library system, all built into the os for free. Imagine a directory that contains “Every metal song I have, from 1989 to 1993, that I haven’t played in three weeks” or whatever else you want.

Back when people used files and all applications were local first, this was probably much more exciting, but it’s still pretty cool.

It's the open source version of BeOS, the original alternative OS for PowerPC Macs.

The excellent answer by @bartydecanter@lemmy.sdf.org already presented the cool features of the file system. There are a bunch of other interesting features found throughout the OS.

Pervasive multithreading and multitasking makes Haiku very reactive and fast, even under load. Back when BeOS came out, the killer demo was playing several videos simultaneously without stutter. This is of course less impressive today, but you can fell this all over the OS when using it.

Window management has two really cool features called Stack and Tile. Enabling you to stick windows together, so they move as one. On top of that you can put several windows from different applications together into one tabbed window bar . It's super cool and unique.

The biggest difference when using it compared to the big desktop operating systems today is that it gets out of your way and just lets you do things. Using it will make you realize how cumbersome the current desktop has become. Of course there are some security downsides, as there's no pervasive sandboxing, rights management, and so on.

Running on real hardware can be difficult because of a lack of drivers. I highly recommend trying it in a VM (VirtualBox, qemy, UTM) first. The increasing number of ports (mostly FOSS stuff you know from Linux) make this operating system actually practically usable. The ports don't take advantage of the Haiku specific features, but are great overall. Especially the KDE apps are a good fit.

Some people say it's ready to be a daily driver even it's still in beta, others say it's what Linux used to be .

If nothing else try it in a virtual machine for sure. Peripheral support can be spotty. One of the bigger hitches for me was getting a relatively up to date browser binary installed. I hear it's getting better. I ran BeOS on a Pentium II back in the day. It was awesome.