A nifty little image that's been circulating the 'net of late might pique the interest of that Valve fanboy/girl in you. The image in question is a massive infographic documenting each voice actor to work on a Valve game, including which characters and games they were in. Seriously, this thing is so huge, our systems couldn't even handle the original version.
The graph is broken down by games, including Dota 2. Not surprisingly, most of the voice actors provide voices for more than one champion.
Keep in mind that one or more characters might be missing from the graph; I can't for the life of me find Skywrath Mage on there. Even so, it's worth checking out, whether you're into Dota 2, Team Fortress 2, Portal, or Half-Life. Yep, it has the actors for TF2 as well.
All credit goes to the creative-minded Michael Godlewski. You can check his blog for the full-sized version, including a PDF download.