Here's what I know:
Italeri and Testors have had a close association for years and do trade molds with each other. Additionally, Italeri is reboxing a load of old Esci kits lately, they've also been known to rebox Bilek and Zvezda kits, usually in the case of Bilek and Zvezda kits its clearly marked on the Italeri box what the kit origins are. As for the relations between Dragon and Italeri, if you ever see the 1/72 Sukhoi SU-24 Fencer aircraft kits in the Dragon packaging, they do originate from lesser known Italeri molds.
Revell Germany does sometimes exchange molds with the parent Revell Monogram, RG also holds most of the old Matchbox molds as well. They also have some old Frog molds.
Airfix and Heller are another long standing association, both being owned by the Humbrol company. Quite often you will see molds being exchanged between the two companys. It my understanding that when Airfix wanted to make some kits to mark the anniversary of the Falklands War, they got their hands on some Heller molds of a 1/48 Dassault Super Etendard to represent the Argentine aircraft that sank the HMS Shefield. Its to my understanding that Airfix's "new" 1/48 SAAB Viggen is just a reboxing of Esci's old molds.
As for AMT, they seem to have largely gotten out of the plastic kit business, but some of their molds have shown up in AMTech kits. They did have an association with Esci for a while, but thes days it appears as though Italeri has the lion's share of the old Esci molds.