Best places for live music in Atlanta?

Depends on what type of music you like. Check out the calendars for Terminal West, Variety Playhouse, Aisle 5, The EARL, and Smith's Olde Bar to name a short few. There is music on every corner.

Seth's answers are good, but I would add Eddie's Attic, Tabernacle, The Loft, The Masquerade, and definitely Grocery on Home. The Masquerade is an iconic Atlanta venue located in an old historic mill that is slated to be torn down soon, so see a show there while you still can! Grocery on Home is a really neat venue located in a former Grocery Store (hence, the name) in the Grant Park neighborhood. The musician roster is carefully curated by owner Matt Arnett, who also curates musical evenings at the High Museum of Art and the aforementioned Eddie's Attic.