A to Z? Ok...
(3.5) Acid Arrow
Not a very good combat spell, Acid Arrow does shine in specialized circumstances.
Unlike Magic Missile, this spell allows you to target anything, not only living creatures. Dealing 2d4 acid damage per round, it will melt through anything weaker than wood very easily (though personally I would rule that glass is immune to acid, despite the low hardness). While this isn't terribly useful for PCs, its huge range means that a rat bastard DM can use it to damage or destroy the PCs' equipment in surprise attacks as soon as they put it down.
The Spell Specialization feat (from T&B) increases the damage by 2, bringing it to 4-10 - but I'm not sure whether it would increase it on the second and subsequent rounds as well. If so, it would be a pretty good combo.
Acid Arrow is one of the preciously few direct damage spells that ignore spell resistance. This is a very noteworthy quality; but since so many outsiders also have acid resistance, it could be worth getting some way to turn the damage into, say, sonic (via metamagic or Archmage PrC ability, for example).
Putting an Acid Arrow on a caster who lacks the Concentration skill will be a major hindrance, forcing a DC 12 (or so) check for any spell or spell-like ability. Examples include low-DC outsiders and fey.
Finally, a Persistant (T&B) Acid Arrow does 28800d4 damage, for an average 72000 damage, on a 6th level slot, no save or resistance, just wait, and if that isn't nasty I bloody well don't know what is. Actually, no. I was wrong.