Depends on what Version of D&D you are talking about... 3rd edition has no racial penalties for multi-classing. In fact, it has racial bonuses for it, with how the experience point system is set up to penalize multi-class characteras that have more than 4 levels of difference between their highest and lowest levels. Each race has a favored class that is not counted for this experience penalty. Humans just use their highest leveled class as their favored class.
Multi-classing a wizard or sorcerer is IMO generally a bad idea because they only get stronger at higher levels. Now, you can add levels in these classes to a character that started as something else if you want to (such as fighter), but it is a good idea to keep a character started as a wizard as a wizard. The same goes for clerics and druids.
Multi-classing a fighter past about 10 levels is IMO generally a good idea, because they don't get any cool feats at high levels, just more bonus feats. And you can usually get all you need with the feats you have gotten until 10th level, plus those you get for whatever other class you take. Maybe take levels in wizard or cleric to get those level 2 and 3 stat boosting spells to help out with the fighting abilities. Of course, many classes can benefit from 4 levels in fighter because that is the only way to get the weapon specialization feat.
Multi-classing a bard is, quite frankly, fairly stupid (except for 4 levels in Fighter to get weapon specialization), as they are essentially a multi-classed character already.
Paladins might be able to benifit more spell-wise by taking levels in cleric instead of Paladin past a certain point.
Of course, it all boils down to the party you are in. If you have no thief, someone multi-classing a few levels in thief can be a great asset to get some trap and lock removal abilities. Same goes for lack of a cleric and some basic healing spells.
[ June 18, 2004, 04:58: Message edited by: Imperator Fyron ]