CDDL
Sun's weak copyleft licence from 2004 — and the reason ZFS still isn't in the Linux kernel.
The Common Development and Distribution License is based on MPL 1.1 and draws the copyleft line at the file: change a CDDL file and you must publish the change, but other files in the same program can carry any licence. Sun created it for OpenSolaris, and released ZFS and DTrace under it.
The CDDL and GPLv2 impose requirements that can't both be met, so CDDL code can't be merged into the GPL-licensed Linux kernel. Whether that was deliberate is still argued over, but the result is plain: ZFS on Linux lives on as a separate kernel module in OpenZFS, and when Canonical started shipping it with Ubuntu in 2016 the Software Freedom Conservancy objected. Linux, meanwhile, got Btrfs.