MTG Help: Mana Leak and similar cards...?

Anything that costs mana to play (or has 0 in its mana cost) is a spell that can be countered by cards like Cancel, Counterspell (ironically), Mana leak, Mental Misstep, Spell Pierce, Force of Will, Chalice of the Void, Pact of Negation, Negate, Syncopate, Stumble Upon, Memory Lapse, and Mana Drain. Creatures that are played from the hand are counted as spells before they come into play so are perfectly legal targets for anything that counters spells.

Mana Leak Mtg

Yes. The basic definition of "casting a spell" is paying the mana in the top-right corner of the card to move it from your hand to the Stack. There are other ways to cast spells, such as Rebound, Cascade, and the third ability on Chandra Ablaze, but these all specify that you are casting the spells.

If you want more proof that creatures are spells when they are on the Stack, just look as Essence Scatter.

yes, thats what counter cards do, any card you play is considered playing a spell except for lands