Ultraforce/Avengers #1, and during a period of dimensional instability heroes from various realities come together to fight alongside one another in one spectacular double page spread. From the indicia, these are all authorized cameos. 

As well as Avengers (and some alternate reality versions of same) and Ultraforce (ditto), there's also:
Ash from Event Comics
Mighty Magnor
Several of Dan Brereton's Nocturnals
Barry Windsor-Smith's Adastra, Axus, Bron and Strangehands
Some of Tekno Comics I-Bots
Rick Flagg (American Flagg)
Edge from Bravura Comics
Cyberella
Characters from Epic's Dragon Lines
Sachs and Violens
Black and White
AX (Man Called AX)
Jim Starlin's 'Breed
Groo
a couple of characters from George Perez's Crimson Plague
and more not yet identified.
A more detailed breakdown can be found here:
http://www.marvunapp.com/Appendix7/hardcase-hawke-uv.html#various

 

Now's Sting of the Green Hornet miniseries from 1992 includes full cameos in issue #2 by Steve Rogers, Nick Fury, and Dum Dum Dugan.  Issue #3 also (if memory serves) includes a reference to Namor, but he doesn't actually appear.  If this series is compatible with 616 continuity (there's already a 616 version of the Lone Ranger, so why not his great nephew the Green Hornet?), it would be fun to have an easter egg reference to it in one of their handbook updates (probably not Cap or Nick, since they have so many appearances, but maybe Dum Dum?).

The multiversal Kevin Bacon thread has mentioned a few links in comics related to tabletop rpgs, but I thought it might be worth covering them in a little more detail here.

The house magazine of TSR, publishers of Dungeons and Dragons, was Dragon Magazine, and over its lengthy run had multiple comic strips at the back, ranging from single panels to multiple pages. In Dragon#359, the final print issue (before it went to being purely digital), one of those strips was Order of the Stick, and they included cameos for many of the other ongoing strip the magazine had published:

Top panel, bottom left is Finieous Fingers, who appeared in the early issues of the run.
[img]https://images-na.ssl-images-amazon.com/images/I/51ZyQguZRTL._SX258_BO1,204,203,200_.jpg[/img]
Above him is an armored female and a long-snouted individual being flung through the air - these are Snarf (long snout) and his girlfriend Telerie (armored woman) from Snarfquest.
[img]https://images-na.ssl-images-amazon.com/images/I/610m1Grl9vL._SX383_BO1,204,203,200_.jpg[/img]
Being knocked to the floor by the passing dragon, and then standing back up in the next panel are Phil (Foglio) and Dixie, from Foglio's strip What's New? with Phil and Dixie.

The smoking dragon standing next to Phil and Dixie in the second panel (and also visible, if partially obscured, in the first) is Wormy from the strip of the same name, another from the earlier part of Dragon's run.

The group riding the larger dragon are the strip's stars, the titular Order of the Stick

Beneath the dragon in the first panel you can see the top of the heads of The Knights of the Dinner Table

And at the right hand side of the top panel, trying to get out of the dragon's path, is Yamara

Meanwhile, in other Nodwick strips we see Nodwick and his team of adventurers meet:
Raistlin from Dragonlance, Elminster from Forgotten Realms and D&D creator Gary Gygax

That's not Gary Gygax's only appearance in Nodwick either. Nodwick then connects to another D&D realm, the horror themed Ravenloft and its most infamous resident, the vampire Strahd.

Nodwick creator, Aaron Williams, also created PS238, about a school of young superhumans; in PS238 #39, some of them dimension/time travel, and run into Nodwick's band of adventurers.
Back in the Nodwick strip, we have yet another crossover with a TSR property - the cast of the Dungeons and Dragons cartoon!
 

Loki wrote:

Meanwhile, in other Nodwick strips we see Nodwick and his team of adventurers meet:
Raistlin from Dragonlance, Elminster from Forgotten Realms and D&D creator Gary Gygax

That's not Gary Gygax's only appearance in Nodwick either.

As mentioned, Gary Gygax appeared again in the same episode that the characters from the D&D cartoon turned up in. After someone warned Nodwick's group to prepare to "face the wrath of Gygax!", we got:

With just the initials, it might have been considered an Easter egg cameo, but the name clinches it as a proper cameo.

I also missed some more cameos in the story where Raistlin takes on Elminster, as both drag in their patron goddesses and Raistlin also calls on another ally, adding to the mix:
Mystra

Takhisis and Dalamar

all three established characters from the Forgotten Realms and Dragonlance series.

One last for the moment, which is a combination of proper cameo with Easter egg. In Dragon Magazine #298, Nodwick's team draws the ire of the evil spider goddess Lolth (an established AD&D character); to placate her, they strike a deal, resulting in what I presume is an unofficial but very recognizable guest appearance linking Nodwick and co to Marvel:

2000 A.D. Prog 24 has the story Tharg and the Intruder; I've already mentioned it in the thread for Easter egg cameos, thanks to a number of erstaz Marvel characters appearing, but it has several proper cameos too, providing what I think might be the first linkage between both strips within 2000 A.D. and to strips from other IPC/Fleetway titles:

So we have a link from Tharg to Robot Archie (though at the time this was published Archie could only link (iirc) to the Spider and the Sludge - I don't think he'd had a proper crossover with any other strip yet.
The next page shows Tharg's museum, and that provides a ton of links to the strips of 2000 A.D.'s early strips:

A very incomplete breakdown (more for matching which strips are represented than covering everything depicted):
Top left is the Mekon, linking to Dan Dare.
Below him is a T-Rex, and more distinctively, a Trans-Time skimmer from Flesh!

Below the skimmer is a Lawmaster bike from Judge Dredd:

Moving across from the bike we see a man in a clear tube - that's John Probe from Mach One, during his empowering:

and in the bottom right corner we have Giant's uniform from the series Harlem Heroes:
[img]https://images-na.ssl-images-amazon.com/images/I/61cgz%2Bw8F0L._SX357_BO1,204,203,200_.jpg[/img]
The only strips which had debuted by this early stage that I can't see being clearly represented are Invasion and Shako. 
On the final page we get a stronger link to Judge Dredd in the form of supporting cast member Walter the Wobot: