Factotum
The factotum is a virtually unknown character class in 3.5 found in the Dungeonscape handbook. Unless you like optimization, then you're probably quite familiar with this flavour of cheese.
Basically, the factotum is a super-nerd who uses intelligence and observation to mimic other classes' abilities a couple times per encounter. This guy is quite possibly the best Skill Monkey in the game, has access to every skill in 3.5E including the Iajitsu Focus skill from Oriental Adventures, but is mostly known for breaking the Diplomacy skill. The class can also imitate the class features and spells of other classes, effectively making it the Harry Flashman of the party. One turn you're casting Greater Shout and other high power sorcerer spells, and then you're healing party members like a cleric.
If you're not cheesing, the class is squarely in Tier 3, with immense flexibility but no truly absurd powers. If you want to play a jack of all trades, this is indisputably the best class for that. It also works well with the Chameleon prestige class from Races of Destiny.
The typical class for people who can't make up their mind (and have even heard of it), and potentially the most indecisive badass of all time.