Is it legal to move a piece if your opponent has not hit the clock? I have played some rapid and blitz games in my city,
it was not a tournament, but a friendly match, but there's
something that bother me almost always that I've played:

I move my piece and as my hand is still in the air moving
towards the clock, and even I have not hit it yet,
my opponent has already moved a piece. Is it legal?
Yes, it is.

Let's take a closer look: Article 1.1 of the FIDE rules states:


  A player is said to ‘have the move’ when his opponent’s move has been
  ‘made’.


Article 4 clarifies how a move is supposed to be made. Specifically:


  When, as a legal move or part of a legal move, a piece has been
  released on a square, it cannot be moved to another square on this
  move.


At that point a move has been "made".

Note, however, the provision in article 6.2:


  A player must be allowed to stop his clock after making his move, even
  after the opponent has made his next move.


Even if your opponent already moved, you still have the right to press your clock.