Valve has hinted that a new Half Life may be in the works. Here is a quote from the VP of marketing for Valve, Doug Lambardi:

"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."

Valve is currently working on Portal 2 which will be released this April. It has been rumored that Portal 2 will contain a teaser of the next Half Life.