SoundCloud is a busy sound sharing platform used mainly for music & podcasts. All tracks are streamable, and artists can mark them as downloadable or link to paid downloads elsewhere.
Everything is taggable, and it's easy to bump into music similar to or related to music you like.
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Pro Explore tab has new music constantly
Explore tab on SoundCloud shows trending music that can be filtered based on genre/tag. There's a constant stream of new music.
Pro Subscribe to artists
You can subscribe to artists to be notified the instant they upload a new song or give an update.
Pro Follow people
You can follow people with similar tastes or that often find new music. When they favorite a song, SoundCloud will notify you about it.
Links to related music, based on what you're listening to, can be found in the sidebar.
Pro Free to use
You don't even have to log in or sign up for an account if you don't want to.
There are paid options for if you want to upload large quantities of audio (eg if you're a music label you'll need this), but it's free to download as much and as often as you like.
Pro Creative Commons filter
SoundCloud allows for finding music that is licensed under Creative Commons.
Some music labels, especially newer ones, release on SoundCloud before anywhere else.
Con Audio ads in some uploaded content
Some content on SoundCloud includes audio ads, particularly sponsored podcasts.
(Note that SoundCloud itself doesn't add these - all the audio you're hearing is exactly as it was uploaded.)
Con Interface is full of bugs
The SoundCloud website has tons of problems, including issues with notifications, favorites, and uploading sounds.