Proc

FreeBSD versions prior to 5.0 had a /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 is sitll used for certain debugging programs, including truss. FreeBSD 6.x has moved away from /proc completely, and you need to jump through hoops in order to get it mounted. FreeBSD 7.x will probably be the first release without any trace of /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 usefull is nowhere near that of FreeBSD's sysctl.