The entire Kirkmanverse was created by Robert Kirkman. Although most believe his book Invincible started the universe it was actually Tech Jacket. The Kirkmanverse is primarily composed of heroes who have appeared in books he has written under Image. In addition to this Kirkman has added characters from other Image titles like Shadowhawk and Savage Dragon and all his surrounding characters. Today even books he does not write, such as the past Brit series, are included in Kirkmanverse continuity.
There is some dispute as to who actually is in the Kirkmanverse. Even if Invincible appears in another character's book, that character isn't it the universe unless he or she has actually appeared in one of Kirkman's book. An example would be Invincible's cameo appearance in Dynamo 5. And in an even more complicated note, even if characters have appeared in books of heroes who appeared in the Kirkmanverse they aren't necessarily in the universe.
A list of all of the confirmed titles that are in the Kirkmanverse.