For an exceptionally high-powered campaign of 3rd Edition Dungeons and Dragons, the DM can do more than to allow characters to start out with 32 points for ability scores. They can opt to allow the rules that cater to the munchkin's sweetest fantasies: taking two classes parallel to one another: getting the best bonuses while the lower ones are ignored. Such characters are the famed gestalt characters from Unearthed Arcana, and make for a very high powered game. They are usable for games with only a few players, allowing them to do stuff normally only possible for a party of double the size.
How does it work?Edit
The first step is the same as rolling up any old character: pick a race, roll/buy abilities and stuff. You then pick not one but TWO classes to build your character from! You get the highest Hit Die, Base Attack Bonus, saves and starting gold of the two classes, and use them to build your character with. The reason a gestalt character is so powerful is because you get the Class Features of BOTH classes as you level up! Spellcasting progresses normally but the two are completely seperate from one another: for example your casting as a wizard does not affect your abilities as an archivist. You can even pick levels in prestige classes to level up higher, but this is limited somewhat: you cannot take levels in prestige classes for both classes at the same time, and you cannot pick prestige classes that would advance the spellcasting of two different sources, IE the Mystic Theurge and friends.
The obvious ups to being a gestalt character is that you're powerful as fuck: you are essentially two characters at the same time, which gives you plenty of options even for classes who previously had only limited flexibility. Another usable part of this is that it covers up weak spots that certain classes normally had: suddenly your wizard has a nice hit die, or your fighter gets good saves, or your warmage gets full BAB.
Gestalt characters normally exist in two kinds: the "same but more" and the "double feature". The "same but more" build takes one thing it does well, IE casting, and starts to do the same thing but with two classes, like the afore-mentioned wizard/archivist. The other is the "double feature": two clases that use the same ability but do different things with it, like the bard and the sorcerer.
The downside to playing a gestalt character is that they are only as tough as the best of the two: a fighter//soulknife is only as tough as the fighter, and less tough than an equally-leveled duo of a fighter and a soulknife. Similarly, the action economy keeps gestalt characters from completely dwarfing normal ones, as any given character can only take so many actions in a round. The other thing you should keep in mind is that just because you took levels in a martial class you do not suddenly become immune to the effects of armor that affects your spellcasting. You also only have one set of ability scores and the same number of feats (aside from any bonus feats you pick up). The most common workaround for these limitations is to focus one side on passive abilities.
If you desire to fix these very minimal downsides and acquire even more cheese than playing a gestalt to begin with provides, just play a Dvati. For those not in the know, Dvati is a race from Dragon Compendium that consists entirely of twins that share a soul between two bodies. For LA+1 you literally get to play two characters. That's right boys and girls, you get 2 full-round actions every single round. The downsides of this are that if one of them casts a spell the other doesn't get to act that round, and each twin gets half of your rolled HD on level-up. This is less of a problem when you're playing gestalt and can slap a barbarian's d12 HD onto a wizard. Or you could be a super nerd and play a Binder/Rogue or something since you can then flank against yourself, take a full attack for full Sneak Attack dice, and then do... whatever it is the binder levels are doing for you. Call lightning I guess.
The Hall of FameEdit
Here's a short list of combinations you might like to tryif you were to play a gestalt character.
Stand back and throw ALL THE SPELLS. Less powerful than the Erudite pick later on this list, but it doesn't require faffing about with Psionics, for which everyone at the table will thank you for. Tends to not be much more powerful than its components, since you're still restricted to the same number of actions and Wizards can't wear armor.
Both use WIS and effects to beat the shit out of people in melee. Combine them to make a crazy powerful CoDzilla who can stomp all the face. And dedicate them to Obad-Hai, because it fits. Like the above, not actually that much better than sum of its parts.
Dragon Shaman//Warmage/Rainbow ServantEdit
A bit of an oddball: both can fight in melee and get profit from high Charisma (casting for the warmage, healing for the Dragon Shaman), plus getting the full 10 in Rainbow Servant as the Warmage will prove to be VERY useful as a winged, firebreathing and spellslugging badass.
This is the Spell-to-Power version of the Erudite, allowing you to pull all Arcane spells from peoples' heads. Meanwhile, your divine part of your brain can learn all Divine spells even easier. This class can as such with a combination of good casting, domination effects and plenty of reading learn every spell in the game. Too bad that they're not very tough in a fight.
An odd combination at first, until you realize you can turn into a Giant, break out a big weapon and beat the everloving shit out of everyone in your way. Or just become a dragon with Fighter feats, for the giggles.
Both of them can do one thing well: shooting bows. Combining the Ranger's tricks and limited spellcasting with the Scout's skirmishing and superior hiding skills you can get a very potent and dangerous archer, picking people off with their bow like it's nobody's business. This was later made into an official hybrid pesudo-class with the "Swift Hunter" feat, which allowed Ranger levels to stack with Scout levels for Skirmish damage.
With monsters being a "class" in gestalt, combinations like these become possible. Succubi gain an innate +16 to charisma which is the same as most starting caster's entire score. Through a few levels in Blackguard they can add their CHA to their saving throws. By taking the Swordsage class and Ascetic Mage feat from Complete Adventurer, she can add her CHA to her AC as well. Throw in CHA based spellcasting, the ability to control minds (based on CHA), teleport at will, telepathy, spell resistance and having just enough skill points to be a pocket diplomancer this build will make a fun anti-hero to play. Or a seriously aggravating villain who can turn people against the party with just her honeyed tongue. To put gestalt into perspective, this is considered to be a moderately weak build...
Factotum 8/anything//Any intelligence based classEdit
You know how number of actions is the main limitation on gestalt characters? Factotum 8 breaks that to tiny little pieces by getting an extra standard action if they spend enough inspiration. Factotum also get to add their intelligence to a lot of things, offsetting the other gestalt weakness of only having one set of ability scores. Factotum themselves are balanced around needing a lot of levels to get this awesome ability, but in gestalt they can do crazy shit. Typical pairings are finishing the Factotum side with Chameleon (which solves the problem of only having so many feats in addition to its other merits) or the remaining levels of Factotum (both normal builds for the class). The other side has many options, but the most common are Wizard, Psion, Archivist, and Warblade (with relevant prestige classes).