Has a ps3 emulator already been developed? Is Last of us playable on pc?

Is The Last of Us playable in any possible way on the PC? Nope.
Is there a PS3 emulator? Yes. Yes there is. It's called rpcs3. The website for it is http://rpcs3.net/
Can the emulator play commercial games? Well. Sort of. Kind of. Not really. It can play Sonic CD. Otherwise some games sort of qualify as half playable. The vast majority of games, The Last of Us included, are nowhere near even being able to boot and get to a menu, much less be even remotely playable.

 

The emulator has been in the works for awhile. This isn't all that recent of an effort. It's just recently (as in the past year) it's started making really noteworthy progress towards commercial game playing ability. It's a very, very slow road though.

 

even if the emulator runs PS3 games, you probably need something like a 16 core i7 to run the game in decent speed

 

This too. Considering you need decent hardware to pull off full speed Wii emulation (at least if you want upscaled 1080p glory), as well as PS2 emulation, I don't foresee PS3 emulation being all that feasible for the average consumer for a few years yet. Technology still needs to catch up to our desires.