Half-Life 3 it ain't, but if it's a new Half-Life game you crave then maybe Lambda Wars will do the trick. An RTS spin on the Half-Life universe, Lambda Wars has been kicking around the modding community for a while as HL2: Wars, but now it's absolutely standalone and most winningly, free. The beta has been available since the weekend.
The game uses the Alien Swarm engine, but as a newly standalone offering you won't need a copy of that nor any Source Engine game to run this. As either the Combine or Resistance you can play online with up to eight human or AI players.
Check out the trailer below: