A variety of ways, but probably most commonly by browsing Discogs and rateyourmusic.com. Discogs is great for looking through labels that release the music you already like, finding side projects of your favorite artists, or just diving into random genres.

Rateyourmusic, is an interesting one. The website as a whole certainly has a certain preferences, but its genre charts are great for getting started with new genres of music and getting a foundation in genres you aren't familiar with.

Last.fm is also a great tool as others have mentioned.