I was, together with Sjoerd Mullender, the grand prize winner of the first
<a href="http://www.ioccc.org">International Obfuscated C Code Contest</a>
in 1984.
This is the program we submitted:

<pre>
short main[] = {
        277, 04735, -4129, 25, 0, 477, 1019, 0xbef, 0, 12800,
        -113, 21119, 0x52d7, -1006, -7151, 0, 0x4bc, 020004,
        14880, 10541, 2056, 04010, 4548, 3044, -6716, 0x9,
        4407, 6, 5568, 1, -30460, 0, 0x9, 5570, 512, -30419,
        0x7e82, 0760, 6, 0, 4, 02400, 15, 0, 4, 1280, 4, 0,
        4, 0, 0, 0, 0x8, 0, 4, 0, ',', 0, 12, 0, 4, 0, '#',
        0, 020, 0, 4, 0, 30, 0, 026, 0, 0x6176, 120, 25712,
        'p', 072163, 'r', 29303, 29801, 'e'
};
</pre>

It was included in books, printed on T-shirts, and all that.
But it's not portable (it only ran on PDP11s and VAXs), and led to a
change of rules that required portability.
Because of this, I recently wrote this one (haven't submitted it yet):

<pre>
char l = sizeof-666,printf(),i[
]="int printf(),o[]={%d,\n:-};int main(){(*(int(*)())&o[1])(printf,"
			"o+1,*o,o+*o);return 0;}\n";void o(

char (*O)(),int *o, int i, char *l){
	for ((*O)( l,i );-- i;)(*O)
		(l+18 ,*o ++); for(
	o=( int*) l, i = 88/sizeof* o;i
	--;)(*O)(l+18,*o ++);(*O )(l+24);
}

int main(){
	i[22] -=':' ;o(
	printf,(int *)o, ((int) main -(int )o+ l-1) / l+2, i);
	return 0;
}
</pre>

Can you figure out what it does (before running it, starting perhaps with
reformatting the code), or how it does it?  Can you find the smiley face
and the unhappy one?  Hints: (1) It's related to the first program.
(2) It's really two obfuscated C programs.
<p>
You may have to compile it using "gcc -fpic" (position-independent code)
in order to make it run correctly.
I've tested it, using gcc without -fpic, on Linux/Pentium,
Solaris/Pentium, Solaris/Sparc, OSF1/Alpha, as well as using
VC 6.0 on NT/Pentium.
I think it'll run on many CPUs with many different operating systems and
compilers.  But, in spite of this, I'm not guaranteeing portability...