One optimization I've always found impressive:

    #include <stdint.h>

    uint32_t testFunction(uint32_t x)
    {
        return ((x & 0xff) << 24) | ((x & 0xff00) << 8) | ((x & 0xff0000) >> 8) | ((x & 0xff000000) >> 24);
    }

compiles into:

    testFunction(unsigned int):
        mov     eax, edi
        bswap   eax
        ret

Another fun one, that only works with clang:

    #include <stdint.h>

    int testFunction(uint64_t x)
    {
        int count;
        for (count = 0; x; count++)
            x &= x - 1;
        return count;
    }

compiles into:

    testFunction(unsigned long):
        popcnt  rax, rdi
        ret