I'm not familiar with pen and paper D&D, so I won't be listing any spells to take from there and put into DDO. Instead, I'll be listing effects which can be caused by illusions and how they can be useful under DDO's combat system.
1) Damage spells that do force/untyped damage, all of which will be will save spells. Basically, these spells will make the target *think* that they are being attacked, so they'll take damage.
2) Increase/Decrease in threat generation via a disguise. Make the caster look like a kobold so mobs won't bother with them, or make your tank look like a hezrou so enemies are more likely to aggro on him.
3) Distractions. Conjure up an illusory lights and sounds to daze, bluff or otherwise distract enemies.
4) Mass version of Phantasmal Killer.