Gestalt characters were first introduced in Unearthed Arcana for 3e DnD (I don't think a 3.5 version was printed, but the concept works across the board). It allows for some very powerful characters in relation to their level, as you're essentially getting the best of everything for two (or technically more, if your DM is truly insane) classes in a single level. For instance, imagine a fifth-level fighter... A gestalt version of this same fighter could combine with rogue. He would still have the same hit points, base attack, fortitude save as the fighter, as well as all of the bonus feats and weapon and armor proficiencies. However, he would gain all of the class skills of a rogue, as well as the increased skill points. He would gain the rogue's sneak attack dice, uncanny dodge, trapfinding, and evasion, and a better reflex save. With the right combos, it can get brutal.
