C

char a;float b,c;main(d){for(;d>2e3*c?c=1,scanf(" %c%f",&a,&c),d=55-a%32*9/5,
b=d>9,d=d%13-a/32*12:1;a=2)++d<24?b*=89/84.:putchar(a=b*d);}

Good news for all International Obfuscated C Code Contest fans -- here's another winning piece from 2013 written by Yusuke Endoh, who is no stranger to the Contest. It is dubbed the Most Tweetable 1-Liner, because is small enough to fit in a tweet (137 characters only), yet can "tweet out a tune", that is, will play some music for your entertainment…

main() { printf(&unix["\021%six\012\0"],(unix)["have"]+"fun"-0x60);}

Just one line of code, but lots of confusion.  What does this program do?…

main[-1u]={1};

You have heard of "zip bombs" (a tiny ZIP file that decompresses to multiple gigabytes) and "XML bombs" (small XML file abusing the entities to consume lots of memory), and now there is a "compiler bomb" to follow suit.  The idea is quite similar -- the source code is only 14 bytes, but the generated executable will be over 16 GB in size…

This obfuscated piece of C code prints the map of India to the standard output.

#include <stdio.h>
main()
{
    int a,b,c;
    int count = 1;
    for (b=c=10;a="- FIGURE?, UMKC,XYZHello Folks,\
    TFy!QJu ROo TNn(ROo)SLq SLq ULo+\
    UHs UJq TNn*RPn/QPbEWS_JSWQAIJO^\
    NBELPeHBFHT}TnALVlBLOFAkHFOuFETp\
    HCStHAUFAgcEAelclcn^r^r\\tZvYxXy\
    T|S~Pn SPm SOn TNn ULo0ULo#ULo-W\
    Hq!WFs XDt!" [b+++21]; )
    for(; a-- > 64 ; )
    putchar ( ++c=='Z' ? c = c/ 9:33^b&1);
    return 0;
}

Quines are computer programs that accept no input and write its own source code on the standard output.  This entry was taken from the Jargon File, the author is unknown.

char*f="char*f=%c%s%c;main()
{printf(f,34,f,34,10);}%c";
main(){printf(f,34,f,34,10);}

This clever program written by Brian Westley calculates π by looking at its own area.  It is one of the winning entries of IOCCC in 1988.

#define _ -F<00||--F-OO--;
int F=00,OO=00;main(){F_OO();printf("%1.3f\n",4.*-F/OO/OO);}F_OO()
{
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}