The only "official" knife for the Marine Corps is the M9 Bayonet. As I remember there were some Ontario K-BAR's in the system (I recieved an inscribed one as an award). So, in my experience/opinion the answer is going to be: whatever they want.
Now to further cloud the issue, some commands (yes, in the grunts)would prohibit the carry of large bladed personal knives. If you owned any, they were "supposed" to be kept in the armory and not in the barracks. Mucho BS, I know.
As far as what is standard for a Marine infantryman to carry would be:
1.) Gerber multiplier, Leatherman, etc.
2.) Tactical folder
3.) K-Bar, or other large fixed blade.
It all depends on the individual. I used to carry a Gerber tool, then either a Benchmade CQC-7 or Balisong, and sometimes a Gerber Guardian II or a CRKT Sealtac. When I was in Cuba, we had latitude, my Marines carried everything from Cold Steel Vietnam Tomohawks, to machetes, to basic lock back folders.
So, to finally answer your question; nothing and everything.
Andy