The 70's and 80's..and, yes, even the 90's were filled with those instances. In the 70's and 80's, Marvel really tried to get new comics more interest by having their main stars make appearances in the other comics.
Let's look at three of my favorite comics from the 80's that I had subscriptions to: Dazzler, Rom, and Micronauts.
Dazzler: Issue 1 had most of the Marvel Superheroes in that one: Spidey, X-Men, Iron Man, Thing. then, you see other characters, like Hulk, Power Man and Iron Fist, and Angel, making appearance in later issues. The next bunch of issues had villians which are more famous from other comics: Dr. Doom, The Enchantress, Absorbing Man and Galactus.
Rom: Rom became very big when he had the X-Men show up in 2 issues. His was pretty popular, I think...and, I can't remember how many characters showed up in his comic, but I remember him making an appearance or two in other comics, like Power Man and Iron Fist. Also, Galactus showed up in his comic, too.
Micronauts: I remember Swamp Thing, Nightcrawler...and, then they even had a miniseries where they teamed up with the X-Men.
Plus, during this time, Marvel Team-Up and Marvel Two In One was real popular for me.
90's was huge, like you said. Remember all the UK books? They got popular because they had the big names in Marvel in those books. (Death's Head II) Plus, other comics that were new, like Ghost Rider and Deathlok is filled with those special guest stars.