The Next Big Thing(s): newqueue, MPsafe

OpenBSD 5.5: The new traffic shaping system (to replace ALTQ, which goes away totally in 5.6) with a new, friendlier syntax. Filtering with received-on any now possible. The block policy in the default pf.conf is now block return.

OpenBSD 5.6: ALTQ removed from OpenBSD (even the oldqueue transition trick).

(Also, OpenBSD IPv6 configuration defaults changed: -inet6 is the default, IPv6 addresses (including link-local) set only when explicitly enabled, global accept_rtadv sysctl removed. No PF syntax change needed though.)

OpenBSD 5.7: carp(4) configuration now requires explicit carpdev

OpenBSD 5.8: spamd(8) now uses divert-to (local only, faster), we got divert-reply option for raw sockets, misc bug fixes

OpenBSD 5.9: General MPsafe movement in pf and the network stack, misc bugfixes, support for old-style set debug statements, see the upgrade guide.