Not sure if I have any sites or apps that I would deem as valuable, but here are a couple I find useful:
Authy is a pretty sweet app that works on multiple platforms (desktop, mobile devices, etc.) and keeps your two-factor authentication accounts synced. I use it every day multiple times a day.
1password has been mentioned, and I use a similar one called
Lastpass. Like Authy, it syncs across multiple platforms, giving you the ability to log into your accounts from anywhere. I'm not sure if 1password has this, but lastpass can generate secure passwords of any length and complexity for you.
Slack is another one that I use all day every day. It's a great tool for team communication and has nice features for joining other teams or inviting outsiders to your team on a limited basis.