You know, I always used to have a tough time striking a balance finding music for working that walked the fine line between "concentration" and "distraction"...
this article has a list of several sites that have background noise generators, like "rain" or "coffee shop".
I never understood why certain music was great for working, and other music wasn't...until I found an interesting explanation at a site called Focus At Will. They hypothesize that while we're tempted to listen to our
favorite songs for working (to pump ourselves up! or to get in the zone!), those songs can elicit an
emotional or sentimental response in us that is actually distracting, because energy that
should be spent focusing on work is instead being diverted to our emotional or memory-processing centers.
Sorry for the nerdy explanation

but I figure y'all are a group of smart, curious folks who might find this interesting!
I actually ended up signing up for the paid service at Focus At Will (I have zero connection to them; I'm just a huge fan). They develop the music themselves to strike that balance, and has all types of music (classical, fast-paced dance, "spa", etc.), water sounds, coffee shop sounds, etc. You can also change the tempo of the music to match your mood. The one downside of it is that it's only free for 15 days, and then you have to pay like $5/month

I'm pretty cheap, but I do pay for it b/c it has helped me a lot, and I tell people about it IRL all the time, like once a week!

I'm writing all. the. time. -- and it's been a life-saver!