All right, here's some metamagic I probably won't take, but might consider it, simply for sheer coolness-
Explosive Spell (Complete Arcane) - +2 Spell Level - A spell that has a Cone, Cylinder, Line, or Burst that allows a Reflex save will now push any creature that fails its Reflex save out of the area of effect. For every full 10’ a target is moved, he/she takes an additional 1d6 damage, plus an extra 1d6 if he/she strikes a barrier. In addition, the target is knocked prone.
Consider this with a Widened Fireball. True, you would sacrifice Maximize Spell, and this Widened Explosive Fireball would be a level 8 spell, but with a 40 ft. burst, a fireball launched directly at an enemy would explode, dealing normal fireball damage (let's say 10d6), and knock them back roughly 40 ft, prone. So, we have our 10d6 ball-o-fun, which would do 60 damage with Maximize Spell, thus being 9th level. With Explosive Spell, however, it would do (on average) around 30 damage, but that extra 40 ft. knockback would add 4d6, thus bringing us up to 14d6, and an average of 42 damage, in addition to knocking the target prone. This would give room for another tiny little metamagic improvement, such as Enlarge Spell, or Blistering Spell. In addition, this could fling the target into a hazard, making for a quick kill. Even if the target's flight is obstructed, they get nailed with 1d6. Which best-case scenario is preferable:
60 damage, 40 ft. radius burst fireball
OR
14d6 damage (average 42, maximum 84), 40 ft. radius burst fireball that knocks opponents 40 ft. away, knocks prone
In addition, consider the Blistering Spell ([fire] spells only) metamagic and the Archmage's Mastery Of Elements and Spell-like Abilities. Would an Archmage be able to prepare a Blistering Fireball as a spell-like ability, useable twice per day, THEN change it to a [cold] or [sonic] spell (using Mastery Of Elements) and still have the Blistering effect? As it is an already prepared and determined ability, and, for a sorcerer, Mastery Of Elements is applied during the casting of the spell, and as the spell is already cast and prepared (as a Spell-like ability), would the Blistering effect remain, dissappear, or render Mastery Of Elements unable to function on that particular spell-like ability?