Seeing that part of Giant Bomb's community is getting more interested in Dota 2 I figured I would list some notable Dota 2 streams along with a brief description. Know that even if you don't catch these streams live pretty much all of them have VoDs that you can watch instead.
Dendi: Stream - Dendi plays for the team Na'Vi who is most known for winning The International 1 and coming in 2nd place at The International 2. Dendi generally plays the solo mid role for Na'Vi. His stream usually doesn't have much commentary and he is almost always in solo queue all pick matchmaking. If you want to see how one of the best solo mid players does it this is a great stream to check out.
SingSing: Stream - SingSing currently plays for the new team QPAD Pandas. He became well known in the DotA community due to his very entertaining stream while playing for Mousesports. He generally plays the solo mid role, although he will change this up from time to time. He has a goofy personality and there's plenty of laughter going on in his streams. He doesn't take matchmaking too seriously and often tries out suboptimal builds.
iceiceice: Stream - iceiceice plays for team Zenith. He's well known in the DotA community for being a top level troll. He generally plays the solo suicide lane position both for his team and when he streams. He has a very aggressive style, likes to troll in game, and often queues with his teammates. One of my personal favorites.
Merlini: Stream - Merlini is an ex pro player who streams practically every day. He constantly responds to chat questions and analyzes everything he does in the match. In my opinion this is the best stream for beginner level players and should really help them grasp the basic and more advanced mechanics in DotA. Merlini has pro level skills even though he is currently not on a team so expect top level plays on top of the great analysis.
Purge: Stream - Purge is a medium skill level player who is mostly famous for his "Welcome to DotA, You Suck!" introductory guide to DotA. He is also known for casting a fair amount of Tournament matches as well.. He streams pretty often and his audience is generally beginner level players. I personally think Merlini is a better place for beginners to go as he has the actual skill to back up his talk, but some seem to prefer Purge so I figured I would mention him here.
Beyond The Summit: Stream - Beyond the Summit is a great place to watch tournament games with a large variety of quality casters. They cast both European and Asian professional scenes and produce tons and tons of quality tournament casts.
Tobi Wan (JoinDota): Stream - Tobi Wan is most known for running the JoinDota website as well as being one (if not the) biggest caster in DotA. He's not the most analytical caster you'll find, but what he lacks in analysis he easily makes up with charisma and energy. Also, he will often have professional players co-cast with him to counterbalance any lack of knowledge he has.
That's all for now. There are many more streams than this so feel free to list more in this thread. These are just some of my personal favorites.