There are some really nice but non obvious dual-class combos when you get to higher levels.
Here are some of my favorites:
- Beastmaster (12-16th level) => Cleric: Animal Summoning 2 as a 2nd level spell, 12 times!
- Kensai => Thief: Why yes, my first feat was "Use Any Item".
- Assassin => Mage: Poison + Melf's Minute Meteors (or later Energy Blades)
- Assassin => Fighter: Poison + Greater Whirlwind[/list]
As far as Cleric / Mage combos go, in my opinion their best feature is being able to put a Heal spell in a Chain Contingency, and their second best feature is allowing Mages access to half decent shields and weapons.
Anyway, what's everyone else's favorite dual-class combo?