The Most Important Devices

/dev/null
Black hole (empty when read)
/dev/zero
Infinite source of zeroes (black hole when written)
/dev/random
Infinite source of very strong random numbers (needs entropy)
/dev/urandom
Infinite source of random numbers
/dev/tty
The controlling terminal for this process
/dev/mem
Physical memory (reserved to the superuser)
/dev/ports
I/O ports of the PC (reserved to the superuser)
/dev/stdin
/dev/stdout
/dev/stderr
The predefined streams for the current process