Hi SQ2, Buzz here. Here in the United States, it's considered dangerous to ride in the cockpit. So the FAA prohibits normal people from doing so. It's leftovers from the wave of hijackings 25 years ago.
But if you work for an airline, you can politely ask the flight crew. Most of the time they're accomodating. Sometimes they're tired from a 4 day trip and don't want comapny.
Dad used to fly for a living, i rode with him several times before he retired. To be fair, i'm a line mechanic with United so i already had one of the prerequisites.
Yes, it's interesting compared to riding in back. The seat doesn't recline, the stereo's kind of poor, leg room is sometimes questionable. (grin) But the view is worth the price of admission.
OK, what has jump seats: 727's have a pair, 737 and DC-9/MD-80 have a fold down seat that leans on the cockpit door. You mentioned 747's already. 757's have one jumpseat, 767's have one or two depending on what size airplane (200 vs 300). I don't know the 777, they don't frequent where i work. Le Airbus 320 has two. DC-10's have a pair of seats, and I'm trying to remember what our old DC-8's had, just one i think.