It's simple as the title suggests really. A certain Mr Newell is playing with knives in the office. You know every day work stuff. But wait what is that lurking in the background. It's a monitor with the words Half-Life 3 scrawled on it. Is it real? Is it fake? Is it photo shopped? (which I guess would fall under the fake category but nevermind.) They always are working on their arsonal. After Dota and the new Counter Strike I don't think they have anything else on the table. My guess is Half-Life 2 Episode 3, a few hours in to wet your whistle followed shortly by Half-Life 3 a month or two later, with what I'm guessing will be a majorly updated Source engine. Whether they choose to rename it as they did between Half-Life 1 and 2, who knows. As for time line, I'm going to guess next year at the earliest. They have DotA and CS:GO to tweak and play with throughout this year.