It's going to depend, a lot, on what plastic(s) are involved, and what you want out of the glue.
For "general junk stuff", there's a good chance that you're looking at polyethylene or polypropylene, which are actually pretty difficult to glue. The Loctite "Plastics bonding system" is specifically designed for low-surface-energy materials like this. Their "Epoxy plastic bonder" works for other difficult plastics like nylon and Delrin, and ABS which is also pretty common in stuff around the house.
Between these two, there's not a lot of common household "junk" that you won't be able to glue. It is important to be able to identify what you're working with, though...