raw_input without pressing enter

Under Windows, you need the msvcrt module, specifically, it seems from the way you describe your problem, the function msvcrt.getch:

Read a keypress and return the resulting character. Nothing is echoed to the console. This call will block if a keypress is not already available, but will not wait for Enter to be pressed.

(etc -- see the docs I just pointed to). For Unix, see e.g. this recipe for a simple way to build a similar getch function (see also several alternatives &c in the comment thread of that recipe).

Python does not provide a multiplatform solution out of the box.
If you are on Windows you could try msvcrt with:

import msvcrt
print 'Press s or n to continue:\n'
input_char = msvcrt.getch()
if input_char.upper() == 'S': 
   print 'YES'

Actually in the meantime (almost 10 years from the start of this thread) a cross-platform module named pynput appeared. Below a first cut - i.e. that works with lowercase 's' only. I have tested it on Windows but I am almost 100% positive that it should work on Linux.

from pynput import keyboard

print('Press s or n to continue:')

with keyboard.Events() as events:
    # Block for as much as possible
    event = events.get(1e6)
    if event.key == keyboard.KeyCode.from_char('s'):
        print("YES")