Multi-classing can be really powerful, where the abilities of two or more classes synergise, or where you can cherry-pick abilities from one class to match your main one. A half-orc Cleric with 1-3 levels of Barbarian can - especially with judicious application of Righteous Might, Divine Power and Bull Strength turn into a complete combaat monster while still having all his Cleric spells. Fighter/Thief Barbarian/Thief Ranger/Thief combos work very becasue the dex-based skills (e.g. Tumble) and thief abilities (e.g. sneak attack, flanking immunity) make for a more flexible, more powerful fighter. And of course every power-gamer would take a level of Ranger for Two Weapon Fighting 'til they fixed that in 3.5

Fighter/Wizard combos can work too, but not as dual-role Fighters who can be wizards; more like Fighters who boost themselves with a few spells (True Strike, Shield, Expeditious Retreat (not normally used for retreating as such...), flame arrow etc). But expecting a Fighter/Wizard to be both a good fighter and good wizard is sort of missing the point isn't it? If anyone could be both without much penalty, why would anyone be a single class character?

You can also have fun tinkering with strange character designs who take levels in 4 or more classes to build an individual character concept or style. It's not normally a power-gaming option, but it amuses some players...