I watch a lot of GameGrumps. But a few other channels I watch are:
LazyGameReviews (AKA phreakindee): This guy knows how to talk about games. Not only is his voice very appealing to listen to, but he's very insightful, and sometimes even hilarious. http://www.youtube.com/channel/UCLx053rWZxCiYWsBETgdKrQ
VSauce: This channel is already pretty popular. I'm sure a few of you have heard of it before. His early videos are not exactly the greatest, but his new ones are all about facts and learning and he goes pretty in depth. I LOVE learning on youtube. http://www.youtube.com/channel/UC6nSFpj9HTCZ5t-N3Rm3-HA
Razethew0rld: These guys are all about tech, PC hardware, and software advancement. They give great, in depth PC hardware reviews, and also have an amazing half hour segment called The Tek in which they talk about all sorts of things, including insightful news stories about the good and bad uses of technology in recent times. http://www.youtube.com/channel/UCNovoA9w0KnxyDP5bGrOYzg
And I have a few others like The Amazing Atheist, PeanutButterGamer, JonTron, Caddicarus, all great gaming channels.
EDIT: Oh yeah, Classic Game Room!! How could I forget Mark! This guy knows how to have FUN in his games. Shocking, right? He also has a very wide and respectable taste in video games. Its a video game review channel that has expanded greatly. He's very charismatic and fun to listen to when he reviews game, both old, and new.