ULTIMATE CODEGOLF!!! (Codegolf a turing-complete system)

Question

The goal: write a program that implements a turing-complete system. It could be cellular automata, a tag system, a turing machine, an interpreter for a language of your own design... the type of system doesn't matter as long as it satisfies the following conditions:

• Takes a "program" from the user as input (ex. the initial state of the turing machine)
• Runs the program
• Outputs the state of the system (or just the returns the output of the program)
• It's possible to compute anything that can be computed with the right input program to your system.

Self-interpreting is forbidden.

As usual entries in the same language that implement the same type of system will be judged by byte count.

For example: programs which implement a universal turing machine in Python will be compared against other programs which implement universal turing machines in Python. The spirit of the challenge is basically the simplest possible programs that can model a turing-complete system.

Because of the diversity of possible programming languages, vote for entries that you believe are clever, elegant, or especially short. The winner will be the entry with the most votes.

Answer 1

Python 3 for the language ///

A terribly long program, accepts input as a series of lines terminated by a newline and then the EOF character (May be Ctrl-Z or Ctrl-D depending on your OS).

s=''
try:
 while 1:
  s+=input()+'\n'
except:
 pass
try:
 while len(s):
  if'/'==s[0]:
   s=s[1:]
   f=r=''
   while'/'!=s[0]:
    if'\\'==s[0]:
     s=s[1:]
    f+=s[0]
    s=s[1:]
   s=s[1:]
   while'/'!=s[0]:
    if'\\'==s[0]:
     s=s[1:]
    r+=s[0]
    s=s[1:]
   s=s[1:]
   while f in s:
    s=s[:s.index(f)]+r+s[s.index(f)+len(f):]
  elif'\\'==s[0]:
   print(s[1])
   s=s[2:]
  else:
   print(s[0],end='')
   s=s[1:]
except:pass

Try it on Ideone!

It has been slightly golfed.