Valve have been very quiet about any future Half Life titles, with Half Life 2: Episode 3 eventually coming out or possibly skipping it and going directly to Half Life 3. But, Valve Software's Doug Lambardi has said that the team at Valve are not done with the Half Life world or Gordon Freeman.
You will ever see it, yes. We are not done with Gordon Freeman's adventures. I have nothing other than that to tell you today, but hang in there with us.
There we have it, confirmation from Valve that there is something to look forward to with the Half Life series - when we get it will be the biggest question. I personally think Valve are working on Source 2.0 and will unveil this with Half Life 3.