Free Music Archive (FMA) is a music library with thousands of free Creative Common tracks. It includes music curated from radio stations such as WMFU and CBC Radio 3 as well as popular music festivals such as Primavera Sound.
There is a range of music available in nearly every genre. As well as all the popular genres such as pop and rock there are some very specific genres to enjoy such as novelty sound effects and nerdcore (hip-hop music for nerds).
There is a variety of Creative Common licenses used but the search function is very flexible, allowing filtering by allowable use.