Top Ten Best Resources for Discovering New Music

Not even being biased here. I only knew the popular bands like Sabbath, Dio, Priest, Maiden, Motorhead, Big 4, etc. Through this site, I discovered an entire world of metal, punk, and so much more.
Song samples from iTunes is pure genius.

You can find other people's playlists. The related artists tab under a certain band will give you some good similar bands.

My friends and I sometimes make lists of bands to check out and exchange them. It's a great way.

Some people, like Jared Dines, expose underground bands. Under related videos, you can find new bands. Channels exist that specialize in exposing underground music, like Stoned Meadow of Doom (Stoner/Sludge/Doom Metal, Psych/Garage/Stoner Rock).
I run Stoned Meadow Of Doom, and it is a huge channel with over 50,000 subs now. I love it!
They have lists and stories that expose bands to you, whether you know them or not.
You can find pretty much any heavy metal band that ever existed on this site.

It's like YouTube - you can listen for free with no registration, but you can also create an account, buy songs/albums directly from the site, follow a musician, take part in discussions, forums, like or dislike songs, etc. There's also a related tracks tab.
Unlike YouTube, only music artists can upload music (their own music) on SoundCloud, and for that reason, the sound quality is better than on YouTube (on average). SoundCloud is the professional version of YouTube - bands upload their own songs and usually entire albums.
The best part is that very often bands allow you to legally download their tracks for free (usually in mp3 format). It's really easy - just click on the 'download' button (where available), and the song goes to your computer in seconds. Registration is not required for the free download, but you have more rights if you have an account.
Another advantage of SoundCloud is that billions of new, underground, indie music artists upload their songs on this site.
Enjoy metal videos for free 24/7, featuring old and new bands, popular metal bands, and bands you have never heard of - no registration, logins, passwords, etc. Just click and watch at http://www.heavymetaltelevision.com/
A quick Google search can and will expose you to a whole new world of music.
The Newcomers

I like this site. It champions prog and power metal with videos, links, news, reviews, previews, new bands, and upcoming shows. You can also buy CDs directly from the site (online store). There are good deals all the time. This is a very intelligent site run by a metal musician.

Discovered so many great obscure artists from Amazon.

This is how I found a lot of my favorite artists.