Google Play Music Unlimited usually grants new subscribers a 3 month free trial, but for a limited time, the trial period is extended to 4 months. In order to get the offer, simply follow the source link while logged into your Google account. Where the usual subscription prompt would offer you the normal amount of free time with the service, you'll be able to score 4 months. Unfortunately, this does not apply to family plans; clicking the "Other Plans" button in the subscription dialog will knock you down to 30 days for individual and family plans, requiring you to click the link again and go into a fresh tab in order to get the deal on an individual scale.
Google Play Music Unlimited, as the name implies, gives you unlimited access to Google Play Music's entire library. Artists from around the globe, from popular fare like Katy Perry and Ed Sheeran to classics like Metallica, and even lesser-known or foreign bands like Robbers On High Street and Janne da Arc can be found, though it's worth noting that the nature of licensing deals means that no music streaming service can ever have a truly complete library. The main draw of Google Play Music is the ability to upload up to 50,000 of your own songs to listen to from the cloud anytime, and mix them in with purchases from Google Play Music, a feature that's present in the free version as well as Unlimited. You can even download music from the service for offline listening. The catch is that you're given a limited number of device removals and re-authorizations, so custom ROM addicts and frequent phone switchers may want to think twice.
As a bonus, Google throws in YouTube Red with its Google Play Music Unlimited subscription plans and vice versa, which means that taking advantage of this offer will net you 4 free months of YouTube Red, including ad-free YouTube watching and YouTube Red original content. That original content includes shows featuring prominent YouTube personalities, and even exclusive movies made with the help of A-list talent. You can also keep listening to music on YouTube with your device's screen turned off to save battery life. This feature of YouTube Red extends to the YouTube Music app.