- Team Fortress, and Team Fortress 2.
- Dota, and Dota 2
- Left 4 Dead, and Left 4 Dead 2.
- Portal, and Portal 2.
- Half-Life, and Half-Life 2.
If they ever make a continuation to the series, I bet it will be named Half-Life 2: Episode 2: Part 2. And it will run on Source Engine 2, of course.