There have been several reports of unwanted advertising pop-ups with doubtful site urls being spread via "malvertising" in the free version of Spotify.
"Spotify's ads crossed from nuisance over to outright nasty this week, after the music service’s advertising started serving up malware to users on Wednesday. The malware was able to automatically launch browser tabs on Windows and Mac PCs, according to complaints that surfaced online.
As is typical for this kind of malware, the ads directed users’ browsers to other malware-containing sites in the hopes that someone would be duped into downloading more malicious software. The “malvertising” attack didn’t last long as Spotify was able to quickly correct the problem." - Macworld
The malware is affecting Macs and PCs.
So, if you've noticed pop-ups and have gone to any of those sites, it'd be best if you scan your computer for malware...or scan it on herdprotect (https://herdprotect.com/downloads.aspx) where 68 AVs scan your PC. Read about it on their site.
If you come up positive on several AVs (more than one or two) then it would be advisable to look up the malware found and deal with it as recommended.
About free things? They're usually expensive, in the end: "There ain't no such thing as free lunch."
Source:
http://www.macworld.com/article/3128289/security/spotify-ads-slipped-malware-onto-pcs-and-macs.html