Work on Wake On Lan support has started in 8-CURRENT. In revision 1.135 ifconfig was taught about Wake On Lan capabilities by Sam Leffler. This means that drivers can now advertise WOL capabilities to user space and WOL can be configured through ifconfig if the network card driver supports WOL.
WOL support still needs to be implemented for almost all ethernet drivers. The following drivers already support WOL:
To get an up-to-date list of drivers which support WOL, run: grep -l IFCAP_WOL /usr/src/sys/dev/*/*.c
Adding driver support
WOL patches found at http://www.stsp.name/wol/ have some additional driver changes, but use an ioctl-based mechanism for ifconfig/driver communication. The approach that is implemented in 8-CURRENT is ifcap-based, so the driver changes need to be adjusted a tiny bit before they can go into 8-CURRENT. Sam Leffler has patches for this but they have not yet been committed (due to lack of testing?).
If you plan to add WOL support for a card see "How to add wake on lan support for your card" at http://marc.info/?l=freebsd-hackers&m=119602736908444&w=2 Note that the obsolete ioctl-based configuration approach is discussed there, but ifcaps should be used instead.
See here for a version of if_vr which is WOL capable and uses ifcaps:
These updates to if_vr were committed to 8-CURRENT on 11th of March 2008. See revision 1.130 of http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/dev/vr/if_vr.c