Spotify's related artists feature. It's insanely good, most things are on there. Also, stuff suggested by Gill & Beez on the Metal Hammer podcast. I don't get a regular subscription to the magazine so it's handy. I'll admit that their suggestions aren't always to my taste, but they do suggest some good stuff (Volbeat, for example).
Other than that, I've gotten into some really good bands from seeing them support other bands. For instance, seeing Black Spiders support Airbourne or The Plight support Cancer Bats.