I'm talking about a "normal" game. When you're playing with jerks then there's nothing you can do. Well, besides not playing with tem, of course.
Consider a 'normal' non tricked out wizard.
Things he can do in combat:
Deal area damage, catching 3-4 enemies in one standard action effect and taking off something like half their health.
Create a barrier, dividing enemies so the ones on your side of the barrier can enjoy focus fire. Theres also 'aggressive' barriers like Web and Black Tentacles that will remove enemies from a fight when first placed(I assume nobody's dumb enough to go back into one once it's placed).
Instantly remove enemies from a fight, via instant death, knockout, immobilisation, mind control or Remove From Play effects(Plane Shifting someone elsewhere for example)
Create an ally in a fight, via mind control or summoning, one is slow and the other is not, but between reliability its fairly even.
Improve an ally's combat ability with buffs like Bull's Strength, Haste or Polymorph. Some of these buffs work better on certain characters
Improve allied defenses with long duration buffs. Greater Magic Weapon provides extremely good cost savings to the party, as does Greater and Superior Resistance.
Alter party mobility, using flight, teleportation, exotic movement modes and planar travel.
Less than half of these work in conjunction with a mundane type, but the ones that do can be made EXTREMELY effective. If you have a cleric, your party saves money on magic weapons and magic armor as well, plus get access to protection from the more common one hit kill effects.
On the other hand if you play a caster weak, by focusing on blasting, then your party loses out on all the benefits, and worse, meleers are bad with area spells, since they tend to be smack in the middle and have bad reflexes to boot.