Yeah I've been boar-hunting one time. It's actually very popular here in NZ, probably more than any other country.
This was many years ago. Back then I used a western cutlery bowie. Not the best knife for that job and I wouldn't use the same knife again, but I was able to dispatch the boar quickly and as painlessly as possible. It was before I knew too much about modern knives.
The dogs are quite routinly killed in such hunts, so I don't think I'll be going again. (I'm a dog knut--I've got pitbulls) Nothing happened when I went, but it always can.
As Jason said, bayonets seem popular. Here in NZ also Muela knives from spain seem VERY popular for boar hunting. I guess they're OK, looking at how many hunters are using them.
I caught a bastard illegally crossing my ranch a few weeks ago as he was coming back from a failed boar hunt. He had some sort of homemade knife, poorly executed it looked like. Looked like he tried to rehandle an old kitchen knife. Idiot.
He also had a lever action rifle slung over his shoulder, which pissed me off. I don't appreciate armed strangers on my land. The rifle, like the knife, also looked very poorly executed.
Thing is, if your knife breaks on a bone, and you don't have a back up, you're dead. So you better make sure whatever knife you have not only stabs well, but is durable and shock resistent too.
A long pointy strong knife is what you want. CS Traimaster as mete said would be a good option. Although it's a little long and might be a bit unwieldly.
This is no game, and it's not like hunting dear. This is true hunting. If you make a mistake, you die.
Best of luck and have fun!