Nice things you can do with lsof
# Show all active internet connections
lsof -i
# Show all active TCP connections
lsof -iTCP
lsof -iTCP:57123 # Show active TCP connection on port 57123
# Show all active IPv4 connections
lsof -i4
lsof -i4:57123 # Show active IPv4 connection on port 57123
# Show all file descriptors for a user
lsof -u $USER