From Half-Life to DotA 2, Valve's games almost always have really strong voice acting. And that is, of course, because they hire great voice actors.

This helpful guide by Michael Godlewski shows every voice actor to appear in a Valve game, broken down by which games they were in. It's pretty brilliant.

Click to embiggen, as it is quite big and therefore requires quite a bit of embiggening:

(Source: Imgur via Ashelia)