Instant Mastering (varies) 
These are the websites where you can get your music mastered instantly by a computer algorithm,  no mastering engineer involved.  Usually the mastered track ends up  sounding a bit strange, but hey, give it a shot and see for yourself. 

Landr (www.landr.com)
Emastered (www.emastered.com)

Luxury ($75-$150 per track)
If you have a big budget,  this is the place to go. This kind of mastering studio typically feature grammy winning engineers and an impressive list of clients. For example, Sony Records recently did a project with Mercury Mastering. It's surprising that a major label would use an online mastering company, but that just tells you that online mastering companies are no joke. 
Mercury Mastering (www.mercurymastering.com)
Abbey Road Studios (of Beatles fame) (www.abbeyroad.com)

Mid-Range ($45-$60 per track)
You won't find any grammies or major credits here, but you may find some solid work at a more reasonable price. 
Sage Audio (www.sageaudio.com)
Chris Graham Mastering (www.chrisgrahammastering.com

You Get What You Pay For ($25 or less) 
Getting what you pay for isn't always a bad thing. Maybe you'll like it? 
Cheap Mastering Online (www.cheapmasteringonline.com)
Blue Mountain Mastering (www.bluemountainmastering.com)

So what about Sola? 
If you've read the About section on the homepage, you know the answer: we built this company with the belief that we could offer mastering that is just as good if not better than the mid-range companies...but at a ridiculously good price. In fact, we think our mastering creeps into the luxury category, but, you know, just $100s cheaper (if you're doing a whole record). So if you want to master an album with Sola (or just a single track) send us a hello or head straight to the Start a Project page. There you can get a free sample or get started on your project right away. 

And what about genres? 
Most of the mastering services out there work with a variet of genres. They're probably better at certain genres due to experience and the kind of music they listen to. The mastering engineers at Sola have experience mastering hip hop, mastering rap, mastering metal, mastering edm, mastering rock, mastering folk, mastering indie, mastering pop, annnnnd you get the point. You name it, we've mastered it.