Retroospect for Teams
blog.workshape.io
blog.workshape.io
15Five trial referral link: http://ssqt.co/tkdC
Edit: How does 15Five compare to Retroospect, now that I see someone from the team is on here?
Retroospect on a basic usage takes like 15 seconds to fill weekly and only tracks 5 basic data points, allows anonymity and produces charts, breakdown reports, etc... it's a more lightweight product, demands less from users and cost less - quite different kettle of fish.
In my experience teams tend to stop using products that demand too much after little time unless you enforce their use, although these tools are probably a better choice if you want in depth reports.
I'm part of the team that built Retroospect. Teams can be selected to be transparent or anonymous. Users who are invited on anonymous teams will not have their identity associated with the data - you will simply be team member 1. We see this as valuable for exactly the reasons you cited - to ensure that honest feedback is always given - something we feel doesn't always happen anonymity isn't catered for.
We use it in our small distributed team (Transparently) and it helps us figuring our when we have an issue worth discussing in depth.
You can anonymise it on larger teams and use it to figure out what actions may have made your team less happy, etc..