Earlier today, when removing a set of headphones from the main/front speaker port in the rear of my computer, I managed to trip over the headphone cable and broke the adapter ( a Radio Shack 274-870, viewable here for reference ) in the process. I managed to remove all of the parts easily, except one, which has remained stuck.

It is very difficult to see into the individual jacks themselves, but with adequate lighting, I can tell that the main/front speaker port, as opposed to the other five 1/8 inch ports ( for surround speakers and line inputs ) has a ring-like piece of metal fitting just inside of the port and blocking off about half the depth, making it impossible to insert other plugs. The part is very difficult to remove due to its shape and the very tight fit ( so far, tweezers have proved useless ).

Have any of you had experience with removing similar parts from ports before?

if it's a hollow cylinder could you try a drill bit, or a fish-hook?
can you get in from the back?

I have neither at the moment, but will try a drill bit if I can get one small enough. It is unlikely that a fish hook would work, unless it were unusually small, and the inside surface of the jammed component very rough.

The port in question is basically built into the motherboard, and is inaccessible from the other side.

If you can get at the end of it, put a dab of superglue on a small round object, touch it to the broken jack plug and extract. This must be done with care, but a drill bit will just spin the plug in the socket if I am understanding your problem correctly. Good luck.