# List all IPv4 network files
sudo lsof -i4

# List all IPv6 network files
sudo lsof -i6

# List all open sockets
lsof -i

# List all listening ports
lsof -Pnl +M -i4

# Find which program is using the port 80
lsof -i TCP:80

# List all connections to a specific host
lsof -i@192.168.1.5

# List all processes accessing a particular file/directory
lsof </path/to/file>

# List all files open for a particular user
lsof -u <username>

# List all files/network connections a command is using
lsof -c <command-name>

# List all files a process has open
lsof -p <pid>

# List all files open mounted at /mount/point.
# Particularly useful for finding which process(es) are using a
# mounted USB stick or CD/DVD.
lsof +f -- </mount/point>

# See this primer: http://www.danielmiessler.com/study/lsof/
# for a number of other useful lsof tips