Linux Limits

Some application require higher limits on open files and sockets (like a proxy web server, database). The default limits are usually too low.
Linux
Per shell/script
The shell limits are governed by ulimit. The status is checked with ulimit -a. For example to change the open files limit from 1024 to 10240 do:

# ulimit -n 10240 # This is only valid within the shell

The ulimit command can be used in a script to change the limits for the script only.
Per user/process
Login users and applications can be configured in /etc/security/limits.conf. For example:

# cat /etc/security/limits.conf
* hard nproc 250 # Limit user processes
asterisk hard nofile 409600 # Limit application open files

System wide
Kernel limits are set with sysctl. Permanent limits are set in /etc/sysctl.conf.

# sysctl -a # View all system limits
# sysctl fs.file-max # View max open files limit
# sysctl fs.file-max=102400 # Change max open files limit
# echo "1024 50000" > /proc/sys/net/ipv4/ip_local_port_range # port range
# cat /etc/sysctl.conf
fs.file-max=102400 # Permanent entry in sysctl.conf
# cat /proc/sys/fs/file-nr # How many file descriptors are in use

FreeBSD
Per shell/script
Use the command limits in csh or tcsh or as in Linux, use ulimit in an sh or bash shell.
Per user/process
The default limits on login are set in /etc/login.conf. An unlimited value is still limited by the system maximal value.
System wide
Kernel limits are also set with sysctl. Permanent limits are set in /etc/sysctl.conf or /boot/loader.conf. The syntax is the same as Linux but the keys are different.

# sysctl -a # View all system limits
# sysctl kern.maxfiles=XXXX # maximum number of file descriptors
kern.ipc.nmbclusters=32768 # Permanent entry in /etc/sysctl.conf
kern.maxfiles=65536 # Typical values for Squid
kern.maxfilesperproc=32768
kern.ipc.somaxconn=8192 # TCP queue. Better for apache/sendmail
# sysctl kern.openfiles # How many file descriptors are in use
# sysctl kern.ipc.numopensockets # How many open sockets are in use
# sysctl -w net.inet.ip.portrange.last=50000 # Default is 1024-5000
# netstat -m # network memory buffers statistics

See The FreeBSD handbook Chapter 11http://www.freebsd.org/handbook/configtuning-kernel-limits.html for details.
Solaris
The following values in /etc/system will increase the maximum file descriptors per proc:

set rlim_fd_max = 4096 # Hard limit on file descriptors for a single proc
set rlim_fd_cur = 1024 # Soft limit on file descriptors for a single proc