NetBSD has a userspace filesystem api with a FUSE compatibility layer: http://www.netbsd.org/docs/puffs/
FreeBSD has had some work porting FUSE. I'm not sure the status I just remember seeing it as a GSoC project.
Some people are working on FUSE for OpenBSD. The GPL license of libfuse seems to be a problem but the kernel side stuff is implemented: http://comments.gmane.org/gmane.os.openbsd.tech/31704