The Adrenaline Vault Previews Summoner taking a look at progress on Volition's RPG that was just declared gold (story). The following excerpt from the article gives a feel for what differentiates the PC version of the Game from the PS2 version:

What really sets the PC version of Summoner apart from its console-based brother is that Volition has included a multiplayer system. With this system you and up to three friends can take control of the various characters in the game with the intention of playing cooperatively. At this point, there is no cohesive storyline for multiplayer, though there will be a few small quests that groups can undertake. This feature is the one that has kept Summoner under development for the past few months, long after the PS2 version has made its debut.