I'm going to break this down into two parts: the music I use when playing and music I use when prepping games are different (at least in general; there are exceptions to this), but I consider roleplaying to be a mix of prep and conceptualization followed by action, so it's really all part of the same envelope.

(questions and image from autocratik.blogspot.com)

General Qualities of Good Roleplaying Music

Basically, when I'm looking for good roleplaying music, I look for thematic background music or more energetic "prep and psych-up" music.

Generally, there have to be three good qualities that I'm looking for.

First, it has to be relatively inoffensive to listen to for long periods of time. This can, of course, be a little flexible depending on context: I have certain things that I can listen to for ages, and other things that will annoy me after a single listening.

Second, it has to lead your mind toward what you're doing, rather than away from it. Most, but not all, of the music I use when I'm roleplaying is instrumental.

Third, It has to match the general setting of the game either in a notional sense or a mimetic one. By this I mean that the sound is either what I'd associate with the game or something that the characters in the game might enjoy.

Background Music

The background music needs to be something that I can play in the background of a scene, and it's what I tend to use when GMing to give a little feeling of life to the table.

There's also a distinction to be made between table music and background listening music.
I use this as background music when I run Degenesis: Degenesis spotify playlist.

This is a background listening playlist that I put on while playing in online games (but listen to through my own headphones, not playing it for my group), meaning that I can have whatever I want in it. I've made similar playlists for a Vietnam-inspired campaign and generally will put them on to get me in the mood. I can tolerate more distracting elements here than I can elsewhere.

The Degenesis playlist is actually light enough to also be used as at-table music, though it does have a few songs that wouldn't work well for that context.

An example of a playlist that would work better for that sort of thing is this one I've used for Eclipse Phase and Degenesis, which is more of an ambient sci-fi and horror soundscape combining artists like Atrium Carceri and Sabled Sun to make a tremendous background element that won't intrude at all.

Movie and video game soundtracks tend to work okay in this role; more action-packed pieces do well with the volume turned down, but the somber background music of something like Sicario does tremendously well in many contexts. The only problem with this is that you might not find enough stuff that goes well with the game that you're running to create a long enough playlist for seamless play audio.

I know a lot of people like dedicated roleplaying-specific music, and that's not bad, but I'm not a huge fan of it myself in many cases. I use the same playlists for listening when I play video games or just when I'm in the mood for something in particular, so it's kind of important to me to have stuff that stands alone, and while some of the tabletop audio places have good music many have passable interludes that focus too much on "what the characters might hear" and not "what players want to listen to".

Getting Pumped Up

I also make playlists for getting psyched-up or into the mood for the game. These are often done from the perspective of a certain character.

For examples, I present this Eclipse Phase character's list, Varden-Avag's playlist (also check out the post where I talk about Varden-Avag), or this playlist I made when working on velotha's flock.

Sometimes these are suitable to play at the table, but usually they're made for reference's sake, or to help inspire me as I work on a character or setting.

The important thing about these lists is that they either tie into a mood I'm going for or present some music that helps me understand the setting or character I'm going to be playing with.

Wrapping Up

I have a hard time describing exactly what music I use because I use so much of it that it would be difficult to define individually. Basically, I go with what I like to hear, and work from there.