Filesystem to share disks between Linux and FreeBSD

You can use ext2. Support for ext2 has existed in FreeBSD for a while and can probably be considered stable. Of course it is native in GNU/Linux as you know.

You could also use ext3 but without journal and extended attributes (use mount options in Linux /etc/fstab), which would increase some limits.

This is probably much better than using a fs which is not native on any of the two systems, like NTFS and the like.