The program that reverses bytes in two distinct ways (in C++)
Style-wise, objectively improvable:

Avoid leaving dead code

This looks like junk you forgot to delete:

int x(1), y(2);

++x = y;


Comment non-trivial algorithms:

reverse0() is funky enough that I had to squint a bit and take a quick minute to run the code in my head to even have an idea of what it's trying to do. An explanation of what each line is "supposed" to be doing would have been helpful here. 

Either name the struct type or the field name, not both

struct Word
{
    unsigned b1 : 8;
    unsigned b2 : 8;
    unsigned b3 : 8;
    unsigned b4 : 8;
} _word;


You don't need Word for anything, so you should just omit it entirely

Redundant explicit void argument list in main declaration:

int main(void) should just be int main()

Style-wise: opinions:

I don't like using unsigned

Since unsigned generally reads like a qualifier (I know it's not, it just looks like one), I personally much prefer being explicit and using unsigned int instead.

non-style stuff, but important:

don't use non-sized types with hard-coded sizeof() expectations.

Your FourBytes struct assumes that sizeof(unsigned) is 4, which is bad, you should be using std::uint32_t to properly enforce that.

Writing to an union field and reading from another one is illegal

This is surprising to many people as using unions to perform type punning is such a common technique, but it is, in fact, undefined behavior. You are only allowed to read from the last field you wrote to.

That means your reverse1() function is not valid C++ at all.

What's legal is reading and writing from a char (or unsigned char) array obtained from reinterpret_casting the pointer of a standard layout object. In fact, it's the only legal way to do direct memory manipulation of an existing object.

Edit: as requested, an implementation of reverse() using casting to char array:

unsigned int reverse(unsigned int v) {
   unsigned int result = 0;

   char const* src = reinterpret_cast&lt;char const*&gt;(&amp;v);
   char* dst = reinterpret_cast&lt;char*&gt;(&amp;result);

   for(int i = 0; i &lt; sizeof(v); ++i) {
     dst[sizeof(v) - i - 1] = src[i];
   }

  return result;
}