Adhesives?

Depending on how small the part is, try some ordinary super glue. For bigger stuff and stuff that sees more stress, there is an epoxy by west systems that works fantastic. It's pretty expensive and it's a two part system. If that sounds like too much work, try the five minute epoxy that any hardware store will have.

you really should try to determine "what" type of Plastic it is that you want to repair. I would use "Loctite" they make many different adhesives for what you are looking for.
Some epoxies will "NOT" work on plastic. The west system that S7horton mentioned, I have not used but know of others that have and they claim great results. And yes, it is expensive.

Please excuse my ignorance, but how does one determine what kind of plastic one has? Any thougts on adhesives for metal?