I'm curious how plugins could be built in a cross browser fashion. I've been researching how to accomplish this, and it seems that different browsers all have their own methods.

For instance:
Safari and Chrome want me to use webkit.

Internet explorer wants me to use Add-Ons or Browser Helper Objects (why couldn't they just call them plugins?).

So, how is this typically done? Flash, java, shockwave, and embedded pdfs all manage to get this idea off the ground.