I like Arcane Preparation combined with Quickened Spell for Bards and Sorcerers.
This allows them to Quicken spells just like other casters.
Sanctum Spell is good for NPC casters, especially Sorcerers who are enemies of the PCs, when they combine it with Spell Focus and Greater Spell Focus. Metanode Spell (Underdark or Champions of Ruin) is similar in this regard.
I also like Silent Spell for Bards and Sorcerers, Sudden Silent for other casters. Although getting caught up in a Silence or Deafness spell is relatively rare in most campaigns, when it does happen, it's a pain in the butt to lose some or all of your spell casting capability.
For pure damage, only Repeat Spell (Complete Arcane) can beat Empower Spell (tmk). The real balance point with this is that spells increase in the number of dice as is, so it is sometimes better to Empower and other times better to Maximize.
For example: 18th Level Arcane Caster
10D6 Empowered Maximized Fireball (8th level) does a save failed average of 77 points of damage
15D6 Maximized Cone of Cold (8th level) does a save failed average of 90 points of damage
15D6 Empowered Cone of Cold (7th level) does a save failed average of 78 points of damage
18D6 Empowered Delayed Blast Fireball (9th level) does a save failed average of 94 points of damage
24D6 Meteor Swarm (9th level) does a save failed average of 84 points of damage (but the area of effect is larger)
10D6 Empowered Repeat Fireball (8th level) does a (two) save failed average of 104 points of damage (if your opponents do not move in the meantime)
15D6 Repeat Cone of Cold (8th level) also does a (two) save failed average of 104 points of damage (again, if your opponents do not move in the meantime)
10D6 Maximized Repeat Fireball (9th level) does a (two) save failed average of 120 points of damage (again, if your opponents do not move in the meantime)
Repeat Spell is awesome if you find a way to keep your opponents in the same area of effect (wall spells, Entangle, Evard's, Acid/Solid Fog, Web, etc.).
Also, Entangling Spell (Champtions of Ruin) is nice. Especially if you combine it with Repeat Spell.
10D6 Entangling Repeat Fireball (8th level) does a (two) save failed average of 70 points of damage since most of your opponents will not be able to move far as is.
The advantage of this is not so much the damage, but rather the fact that it gives your side time to react to the opponents since many of them will be entangled for two full rounds (plus if only some of the opponents approached on round one and new ones approach on round two into that area, they will get hit with the Repeat spell as well). This also lowers their Reflex saving throw for the spell you cast at them on the second round.
