Complete list of everything that can be reacted to

Short answer: you can react to anything that stays on the Chain as a finalized item. Reaction isn't about "responding" to arbitrary events — it's a permission to play/activate during a Closed State, so a window only exists if something is sitting on the chain.

Reactable:

• Spells — they linger on the chain after finalizing (Rule 359.3), so opponents get priority before resolution.
• Triggered abilities of any sort — "When you play me," "When I attack/defend," "When I move," Deathknell, reflexive triggers, etc. (Rule 383).
• Activated abilities, including Equip and any gear/unit/legend ability with [Reaction][>] or [Action][>] (Rule 818.1). Reactions can also stack on other Reactions.
• Any of the above during a Combat Showdown, as well as combat triggers like "When I attack."

Not reactable:

• Playing a unit or gear — permanents leave the chain the moment they finalize, so no window exists (Rule 359.2). You can react to any "When you play me" trigger the permanent creates.
• Hiding a card — discretionary, no chain.
• Discretionary actions that don't use the chain: Standard Move, Recalls, scoring/Conquer/Hold, combat damage, cleanup steps (FAQ #5996, #2835).
• Add abilities and tapping runes for mana — these finalize and resolve immediately, even mid-resolution, so nothing lingers to respond to (Rule 429.2, FAQ #3799).

One practical note: Reaction isn't strictly "responding." On your own turn you can play a Reaction with an empty chain just like an Action spell; it's only priority that limits you on an opponent's turn (FAQ #11498).