My favorite, copyright me (to the extent allowed), is . . .
Variable Volume Fireball
Evocation [Fire]
Level: Sor/Wiz 6
Components: V,S,M
Casting Time: 1
Range: Long (400 ft. + 40 ft./level)
Area: 20-ft.-radius spread (see text)
Duration: Instantaneous
Saving Throw: Reflex half (see text)
Spell Resistance: Yes
This spell’s default effect is a ball of fire exactly similar to that produced by a Fireball spell except that the maximum damage is 15d6. What sets this spell apart is that the caster may increase or decrease the radius of the spread of the fire. If the radius is increased, the damage to each individual caught within the spread is less; if the radius is decreased, the damage is greater.
For every 5’ diameter reduction of the spread, the damage increases by one die (to a maximum of 20d6). For example, a 15th level caster might choose a 30’ diameter fireball doing 17d6, a 20’ diameter fireball doing 19d6, or a 15’ diameter (or smaller) fireball doing 20d6.
For every 5’ diameter increase in the size of the spread, the damage decreases by one die (to a minimum of 1d6). So, the previously mentioned caster might choose a fireball with a 90’ diameter which would cause 5d6 damage. The same caster’s maximum diameter fireball (doing only 1d6 damage to all within) would be 110’.
The smaller bursts of fire are usually blue or white in appearance, and the larger bursts are somewhat orange or deep red. The material component is the same as for a Fireball.
The really large-diameter fireballs are great for battles, nothing like doing 3 points of damage to a few hundred low-level combatants. The really small-diameter fireballs are great for hitting one or very few enemies, all of whom are near allies.
I'm sure someone will point out all the potential flaws with the spell (comparing it to Delayed Blast Fireball, which is 7th level, or saying how this gets around the point of certain feats). But, this is how I wrote it. I suppose it might be more balanced as a 7th level spell--I dunno, I haven't taken the time to compare it to other spells of similar level.
Dave