Proc

In Linux, you can use /proc to see various bits of kernel and system info. In FreeBSD, the equivalent is sysctl. Linux also has sysctl, but it's usefulness is nowhere near that of FreeBSD's sysctl.

FreeBSD versions prior to 5.0 had a working /proc implementation. However, several serious security issues were discovered and it was decided to move away from /proc. The replacement is sysctl.

FreeBSD 5.x still has a mountable /proc system, and it is still used for certain debugging programs, including truss.

FreeBSD 6.x has moved away from /proc completely, and you need to jump through several hoops in order to get it mounted. No tools included with FreeBSD actually use it for anything, though.

FreeBSD 7.x will probably be the first release without any trace of /proc.