MPlayer runs on FreeBSD, OpenBSD, NetBSD, BSD/OS and Darwin. There are ports/pkgsrc/fink/etc versions of MPlayer available that are probably easier to use than our raw sources.
To build MPlayer you will need GNU make (gmake - native BSD make will not work) and a recent version of binutils.
If MPlayer complains about not finding /dev/cdrom or /dev/dvd, create an appropriate symbolic link:
ln -s /dev/your_cdrom_device /dev/cdrom
To use Win32 DLLs with MPlayer you will need to re-compile the kernel with "option USER_LDT" (unless you run FreeBSD-CURRENT, where this is the default).
If your CPU has SSE, recompile your kernel with "options CPU_ENABLE_SSE" (FreeBSD-STABLE or kernel patches required).
Due to limitations in different versions of gas (relocation vs MMX), you will need to compile in two steps: First make sure that the non-native as is first in your $PATH and do a gmake -k, then make sure that the native version is used and do gmake.
As of OpenBSD 3.4 the hack above is no longer needed.
See the Mac OS section.