Wow. Lots of bad information in this thread already.
Intel developed IA64 for Itanium long before AMD developed AMD64 / x86-64. Itanium's architecture however was nothing like x86. It was more oriented towards the server market that Intel was trying to break into than it was towards desktops.
AMD on the other hand, figured why reinvent the wheel. They just slapped 64-bit extensions onto the x86 that they knew and loved.
It's debatable whether or not this is true, but rumors of Intel developing their own 64-bit extensions to x86 existed before AMD said they were doing this. It's believed that Intel didn't openly persue x86-64 because obviously it would compete directly with the new server market they were trying to create with Itanium. Had Itanium done better, maybe they would have risked it.
But either way, as the concept of x86-64 caught on, Intel was forced to implement EM64T to follow on AMD64's heels lest they be caught without any product for that emerging market.
So it becomes debatable which between Intel and AMD actually pioneered x86-64. Chances are, both worked on different concepts of it at the same time, but AMD pushed it openly where as Intel kept it secret because AMD didn't have another 64-bit platform to protect, where as Intel did.
But either way, 64-bit processors themselves were hardly pioneered by either company, as they'd been used in servers and workstations for long long long before Intel and AMD ever bothered.
And x86-64 is most definately not an operating system. It's not software at all. It's just an instruction set extension allowing for 64-bit wide general purpose registers compatible with the x86 architecture. (And, of course, the hardware to run those instructions.) Operating Systems being software, are completely different (though related) animals.