I find it fascinating that Marvel, who is generally known for using their own original characters or adaptations of non-copyrighted characters (ex. Thor), have actually used licensed characters from time to time in their own main continuity. I can understand the reason for it; I assume that it was done to boost sales by tying the licensed properties with their own established characters, and I assume that re-prints/digital subscriptions/collected editions weren't really a concern at the time so the licenses expiring were just not really a concern. However, I think that with digital subscriptions and collected editions it has become more of an interesting curiosity, especially since Marvel's own characters can't have certain issues reprinted because they feature licensed characters. Here are some 616 licensed characters I can think of:

Conan - I'd say he is by far the most successful licensed character, and in fact Marvel currently have the rights to him and are using him in Savage Avengers, firmly placing him within other Marvel characters
ROM - He appears sometimes as Marvel owns his human persona, but the costume and references to the licensed merchandise themselves are not allowed
Micronauts - I think this is similar to ROM as certain aspects appear but the specifically licensed parts cannot be mentioned
Godzilla & Tarzan - Now these ones honestly shocked me, the fact there is a Godzilla in the same universe as the Punisher, for instance, just seems hilarious to me

There are definitely others, but this is what came to my mind! A shame many won't be collected as I think they'd be interesting reads. Any other ones that interested you? Your thoughts on using licensed characters in the main continuity?