Cards for free hero power

There are currently three cards that have an effect on the cost of a hero power, all from the The Grand Tournament set.

These cards are:

Maiden of the Lake: The card mentioned in your original post. It reads "Your hero power costs (1)." This effect lasts for as long as Maiden of the Lake is on the board.

Fencing Coach: This is the card most similar to what you're asking. It reads "Battlecry: The next time you use your hero power, it costs (2) less." This Battlecry effects the Hero Power itself, so is lost if you change hero powers.

Saboteur: Least relevant, but included for completeness. This card reads "Battlecry: Your opponent's hero power costs (5) more next turn."

In conclusion, there is no way to permanently reduce hero power cost below 1.

As of the Mean Streets of Gadgetzan expansion, a new Priest card called Raza the Chained will reduce your hero power cost to 0 for the rest of the match when the battlecry is activated.

While this card is in priest, it has some interesting possible synergies when comboed with Sir Finley Mrrgglton and Kabal Courior, as you could discover a Coldarra Drake for infinite hero powers with the power you receive from Finley.

There are no cards that make the hero power cost 0 permanently. But the card "Fencing Coach" from the "The Grand Tournament" expansion makes the next usage of your hero power cost 2 less. You can even carry this discount over to future turns.