Some acts, such as dropping items or speaking, are consistently considered "non-actions" in the D&D rules (I'm asking about 3.5e in this question, but it seems that this is consistent among all WotC editions, and possibly the TSR ones as well) -- like free actions, they are not charged against the character's action economy, but furthermore are not restricted to one's own turn in combat. Are these actions allowed as part of a reaction/AoO, by RAW?
The only defined "non-action" is speaking, and the rules explicitly say that it can be done not on your turn:
In general, speaking is a free action that you can perform even when it isn’t your turn. Speaking more than few sentences is generally beyond the limit of a free action.
Everything else, including "drop an item," is defined as a free action. See the Actions in Combat section of the d20SRD. Immediate actions can also be taken not on your turn, but those are a pretty strictly defined set of spells/powers that say "immediate action" on them.
So short an ability that specifies it can be used outside your turn as an immediate action, brief talking and other actions by DM fiat are the only things that fit this definition.