This took me longer to find than it should have, so I figure it's worth making a page. The FreeBSD handbook covers mounting Linux partitions, but is apparently outdated, as using their commands gave me errors.
In my case, I just needed to mount a USB drive formatted in Linux. Although RedHat and clones are starting to use XFS as their default, Debian and its clones seem to default to ext4, which is all I'm covering here.
In my case, I was trying to mount a USB that someone had formatted in Linux. The partition I needed was /dev/da0s1.
First one needs the fusefs-ext4fuse packgage. Then kldload the fuse module and lastly, mount the drive with the ext4fuse command.
pkg install fusefs-ext4fuse kldload fuse ext4fuse /dev/da0s1 /mnt
The above requires root privilege. I'm sure it's fairly easy to make it accessible to non-privileged users, but as I need it infrequently, I haven't investigated.