Emmental
 This is a featured language.
Emmental is an esoteric programming language designed by Chris Pressey in 2007.
Emmental is a selfmodifying language. It is defined by a metacircular interpreter (an Emmental interpreter described in Emmental.) The metacircular interpreter provides an operation which modifies operations of the metacircular interpreter.
Emmental is a stackbased language, but it also has a queue.
Contents
Instruction set
(Emmental's instruction set can be redefined, so what follows is the initial instruction set.)
Instruction  Description 

# 
Push NUL (ASCII 0) onto stack.

0..9 
Pop symbol, multiply by 10, add 0..9 respectively, push back on. 
+ 
Pop two symbols, add them, push back result. 
 
Pop two symbols, subtract first from second, push back result. 
~ 
Pop symbol, push discrete base2 log of symbol. As a special case, the discrete base2 log of 0 is considered to be 8. 
. 
Pop symbol from the stack and output it. 
, 
Input symbol and push it onto stack. 
^ 
Enqueue top element of stack without popping it. 
v 
Dequeue symbol from queue and push onto stack. 
: 
Duplicate top element of stack. 
! 
Pop a symbol and an Emmental program (a string of symbols terminated by ; ). Then redefine that symbol as having the same semantics as that Emmental program in the metacircular interpreter.

? 
Pop a symbol and execute it (with the current interpreter). This is sometimes called the 'eval' operator. 
; 
Push the symbol ; onto the stack.

Examples
Infinite loop
Go into an infinite loop. This is done by redefining 0
to mean #48?
, and then executing 0
.
;#35#52#56#63#48!0
Cat program
Redefines *
(ASCII 42) to mean ,.#42?
and executes *
. This program is an infinite loop that does not observe EOF.
;#44#46#35#52#50#63#42!*
Hello, World!
Output "Hello, world!". This is done by pushing the ASCII representation of every character on to the stack, and then popping them off with the .
operator.
#0#10#33#100#108#114#111#119#32#44#111#108#108#101#72...............
Alternatively, this code performs a loop over the characters (rather than outputting each character individually.)
;#58#126#63#36!;#46#36#!;#0#1!;#0#2!;#0#3!;#0#4!;#0#5!;#0#6!;#0#7!#0#33#100#108#114#111#119#32#44#111#108#108#101#72$
Conditional branching
Emmental doesn't have explicit conditional statements, but the eval instruction ?
can be used to "branch" by executing an instruction that depends on the symbol on top of the stack. The discrete base2 log instruction ~
can be used in combination with eval: it basically maps the 256 possible symbols into 9 "equivalency classes" 0, 1, 23, 47, 815, 1631, 3263, 64127 and 128255.
The following programs display different approaches to conditional branching.
Using the discrete log
Input a character and report whether its ASCII value is congruent with 0, 1 or 2 modulo 3. This is done by repeatedly subtracting 3, and branching with the base2 log.
Readable version  Emmental program 

;''0.#8! ;''1.#0! ; '#'5'9'''2''.'#'2'! '#'5'9'''0''.'#'3'! '? #1! ;'#'3'':'~'?#2! ;'#'3'':'~'?#3! ;'#'3'':'~'?#4! ;'#'3'':'~'?#5! ;'#'3'':'~'?#6! ;'#'3'':'~'?#7! ,:~? 
;#35#52#56#46#8! ;#35#52#57#46#0! ; #35#53#57 #35#51#53#35#53#51#35#52#56 #35#52#54#35#50#33 #35#53#57 #35#51#53#35#53#50#35#53#54 #35#52#54#35#51#33 #63 #1! ;#35#51#45#58#126#63#2! ;#35#51#45#58#126#63#3! ;#35#51#45#58#126#63#4! ;#35#51#45#58#126#63#5! ;#35#51#45#58#126#63#6! ;#35#51#45#58#126#63#7! ,:~? 
For better readability, the program has been written using the notation 'A
for #65
(and so forth with every ASCII char). This is not a feature of the Emmental language, it is merely a convention adopted in this article to help understand the examples. With this notation:

'0
means#48

''0
means'#'4'8
which in turn means#35#52#56

'''0
means'#'3'5'#'5'2'#'5'6
which in turns means#35#51#53#35#53#50#35#53#54
Note the nested definitions of symbols 2 and 3 inside the definition of symbol 1: those symbols will be defined when symbol 1 is evaluated.
Using implicit modulo 256
Input a character and report whether its ASCII value is odd or even. This is done by multiplying it by 128.

; '^'v ':! ; ''E'. #! ; ''O'. #128! ; ':'+':'+':'+':'+':'+':'+':'+ 'm! ,m? 

#59#94#118#58!#59#35#54#57#46#!#59#35#55#57#46#128! #59#58#43#58#43#58#43#58#43#58#43#58#43#58#43 #109!,m? 
Using massive symbol redefinition
Input a character and report whether its ASCII value is congruent with 0, 1 or 2 modulo 3. This is done by redefining every symbol to "subtract 3 and try again".
Readable version  Emmental code 

; '#'4'8 '. #! ; '#'4'9 '. #1! ; '#'5'0 '. #2! ; '#'3'':'? #3! ; '^':''#'5'9'+'#'3'v'! #4! ; '#'5'9 '#'3 '#'2'5'5'! ',':'? #255! ; #4#4#4#4#4#4#4#4#4#4 #5! ; #5#5#5#5#5 #5! ; #5#5#5#5#5#4 #4! #4 ; ':'#'1'+ 'd! ; 'd'd'd'd'd'd'd'd'd'd 'd! ; 'd'd'd'd'd 'd! ; 'd'd'd'd'd #4 #255 'd! d 
; #35#52#56#46 #! ; #35#52#57#46 #1! ; #35#53#48#46 #2! ; #35#51#45#58#63 #3! ; #94#58#45 #35#53#57 #43#35#51 #118#33 #4! ; #35#53#57 #35#51 #35#50#53#53#33 #44#58#63 #255! ; #4#4#4#4#4#4#4#4#4#4 #5! ; #5#5#5#5#5 #5! ; #5#5#5#5#5#4 #4! #4 ; #58#35#49#43 #100! ; #100#100#100#100#100#100#100#100#100#100 #100! ; #100#100#100#100#100 #100! ; #100#100#100#100#100 #4#255 #100! d 
 the symbols
\0
,\1
and\2
are defined to output '0', '1' or '2'.  the symbol
\3
is defined to "subtract 3 from top stack element, duplicate it, evaluate the copy", and the symbol\4
to "pop top stack element and define it to be a synonym of\3
".  the symbol
\255
is defined to "redefine myself to be a synonym of\3
; then take one char from input, duplicate it, evaluate the copy".  the symbol
\4
is redefined to "repeat\4
251 times"  the value 4 is pushed on the stack
 the symbol
d
is defined to "duplicate top stack element and increment the copy"  the symbol
d
is redefined to "repeatd
250 times, then\4
, then\255
" 
d
Note that all symbols are redefined to "subtract 3 and try again" when d
is executed; for that reason, that occurrence of d
must be the very last character in the program — even whitespace are not allowed.
Truthmachine
;#58#46#58#63#49! ,:.:?
Computability class
An Underload implementation in Emmental is available, generated by a Haskell program.
It is not quite an ordinary interpreter: It uses Emmental's metacircular evaluation principles to turn the Emmental interpreter into an Underload one. As such, the Underload program you wish to interpret should be appended to the Emmental one.
It supports all Underload commands, but not all Underload characters: By the necessities of this kind of Emmental programming, 10 byte values have been reserved for the interpreter's internal use. These are all nonprintable control codes.
In any case I believe this proves Emmental Turingcomplete. However it may happen to do so in a way which I have previously disagreed with Chris Pressey on whether it counts or not. :P
I also attempted to minimize the number of initial Emmental instructions used for this, based partially on certain challenges proposed in the Emmental distribution. In particular:
 It doesn't use the queue.
 It doesn't use the ~ or : instructions.
In fact, it only uses the initial instructions #1.?!
(and the special stackseparator function of ;
).
Implementations
The Emmental reference interpreter is written in Haskell and can be found in the Emmental distribution on Github. It also contains some example programs as test cases.
There is also an implementation in OCaml by User:Koen, and a Go implementation.
All three implementations are in the public domain.
See also
 Enema, another stackbased, instructionrewriting esolang
 Mascarpone, a direct successor of Emmental