Ok, I know that multiclassing in 3e is fairly open. What I was wondering is what multiclass combinations are not allowed.

Is it possible to have a cleric/druid? wizard/sorcerer?

I mean the combinations apart from the more obvious alignment conflicts.

Thanks

Everything is possible as long as there is no alignment conflict. You can even try a monk/paladin/wizard combo, which will probably create a character as usefull as the dead bodies you find all over the place but still....
For example I gave one level of paladin to my two sorceresses and now on level 18 their saves are approximatelly 22/19/20

Alignment conflicts prohibit the following combinations:

Druid/Paladin
Paladin/Barbarian
Barbarian/Monk

Everything else should be legal, though not necessarily especially useful. :)

Thanks for that. Has anyone tried a sorcer/wizard then?

It sounds cool in theory, but has a lot of problems in practice. The main thing is that your spellcasting levels don't stack. So you have more low level spells, but don't get as many high level spells. And since you cast at a lower level, you do less damage, with easier saves, and lower spell penetration values than you would as a single class wizard or sorceror.

If you keep them even, you will end up 8/8 Wizard/Sorceror rather than 16 Wizard *or* Sorceror. The latter is clearly more powerful.