An experimental Linux distribution under heavy development makes it possible to use software from other, mutually incompatible Linux distributions, all under one roof.
Bedrock Linux does this without using virtual machines or containers. Instead, it uses a virtual file system arrangement, allowing each distribution's software to be installed in parallel and executed against each other.
Side by side on my hard drive
Bedrock Linux's documentation provides a use example: "One could have an RSS feed reader from Arch Linux's AUR open a webpage in a web browser from Ubuntu's repos while both of them are running in an X11 server from Fedora."
The point, according to the project's creators, isn't simply to pull frivolous stunts, but to solve real-world problems that pop up when dealing with the quirks and eccentricities of multiple distributions. For instance, if an application package is dropped from the repository of one given distribution but is kept in another distribution, it's possible to use the second distribution to gain access to the software in the context of the first.
Bedrock Linux uses virtual file systems to map the contents of various distributions into each other. The setup process involves installing one of any number of common distributions, then "hijacking" it to turn it into a Bedrock Linux installation.
Be prepared for some heavy lifting, though. Right now, the setup process involves compiling Bedrock Linux's userland from scratch in the base distribution, then adding other distributions. Specific scripts exists for Debian-based distros (such as Ubuntu), Arch Linux, Yum-based distros (Fedora, CentOS, OpenSuse), Gentoo Linux, and a number of others, but in theory, any Linux distribution can be added.
No VMs, no containers
Bedrock's approach holds one advantage over running distributions side by side in VMs: The distributions are intermingled directly. There's no overhead from a VM interposed between the elements of different distributions.
Bedrock also contrasts with containers in that there's no need to repackage software in a container to be useful. Software packages in Bedrock can access each other without any container-layer complexity. But the makers of Bedrock are quick to emphasize that VMs and containers set out to solve a different class of problems.
It's possible to argue that containers are developing in such a way that it won't be much longer before they reach a level of convenience and transparency and will be able to duplicate the behaviors provided by the likes of Bedrock Linux.
But experiments like Bedrock Linux experiment have their own payoffs. They work with parts of the Linux ecosystem apart from hypervisors or the components used to drive containers, and these parts are far likelier to remain stable across versions and distributions of Linux. They also don't always have to produce something useful to everyone -- if they pay off for a few or lead to innovation put into broad use elsewhere, the experiment will have been more than worth it.