NetBSD took its roots from the original UCB 4.3BSD via the Networking/2 release and 386BSD.
Frustration at getting patches integrated and releases of 386BSD led to FreeBSD, which concentrated the i386 platform, while NetBSD formed to focus on multi-platform support.
Since then, in addition to the many developers within the project, it has imported changes from many other sources, including 4.4BSD Lite. NetBSD has also been used as the basis of other derivatives, including the University of Utah's Lites (on top of Mach), and Apple's Rhapsody.
A more complete timeline is distributed as part of NetBSD in src/share/misc/bsd-family-tree.