I really like their VPN, in fact, I put my entire home network behind Proton VPN. I also use Proton Calendar for shared calendars, for my personal stuff I use a self-hosted solution. Sometimes I upload something to Proton Drive. But probably the most useful Proton product (other than Mail) is SimpleLogin which you also get Premium access to when you subscribe to a paid Proton plan. It allows you to generate an unlimited amount of alias email addresses and all messages are forwarded to your inbox, it’s awesome, I love it.