In my opinion:
OpenBSD - Focused on security above all else. Host project of OpenSSH and pf/carp/altq?. Lags behind on architecture support and performance (Previously poor SMP performance?). Primary use: Router/firewall.
FreeBSD - All around features and performance (zfs,dtrace,pf,linux syscall emulation layer). The most popular, and I believe has the most development effort. Popular freebsd derivatives: pfSense (firewall appliance), FreeNAS (zfs storage appliance), PC-BSD (packaged up for an easier desktop experience). Primary use: Desktop/server/firewall/storage/database.
NetBSD: Focused on architecture support. Supports 57 platforms/15 processor architectures: http://www.netbsd.org/ports/. Primary use: support on embedded/uncommon hardware.
DragonFlyBSD: Interesting technologies being developed: HAMMERfs (compare with zfs), application snapshots, virtualized kernel. Matt Dillon's fork of FreeBSD adding the spirit of AmigaOS. Primary use: Compute clusters?
This is actually a good summary, but it's missing the reason for the DragonFlyBSD fork: light-weight kernel threads. But yes, they've been doing a lot of work around clustering and performance in general.
> At peak hours as of 2007 the database handles around 2-2.5k transactions per second, which generate roughly 40,000 input/output operations per second on the disks.
http://mars.jpl.nasa.gov/msl/multimedia/images/?ImageID=4282