lsof is a utility which gives information about open sockets/files/pipes on many unix systems. You can easily install lsof and try it on your freebsd installation by using pkg_add command.
# pkg_add -r lsof
But FreeBSD has two utilities coming bundled with default installation. They are called fstat and sockstat.
fstat tells you which user,command and pid opened the file, which mount point the file is and information about the open file descriptor like read/write, inode number and mount point of the opened file.Please see man page of fstat for other options.
sockstat gives you information about the opened sockets like which process/command is using it, user of the process,pid,protocol like tcp/udp,stream,dgram,etc.. and connected ports of local and remote servers.