Linux emulation in FreeBSD has reached a point where it is possible to run a large fraction of Linux binaries in both a.out and ELF format. The linux emulation in the -STABLE branch is capable of running Linux DOOM and Mathematica; the version present in FreeBSD-CURRENT is vastly more capable and runs all these as well as Quake, Abuse, IDL, netrek for Linux and a whole host of other programs.
There are some Linux-specific operating system features that are not supported on FreeBSD. Linux binaries will not work on FreeBSD if they use the Linux /proc filesystem (which is different from the optional FreeBSD /proc filesystem) or i386-specific calls, such as enabling virtual 8086 mode.
To tell whether your kernel is configured for Linux compatibility simply run any Linux binary. If it prints the error message
then you do not have linux compatibility support and you need to configure and install a new kernel.
linux-executable: Exec format error. Wrong Architecture.
Depending on which version of FreeBSD you are running, how you get Linux-emulation up will vary slightly: