mQlicker is a free student response sysem which has a very nice interface. Responders use an app that can be downloaded for iOS and Android devices, or from any web browser. Students do not need to create an account, you provide them with an access code to join the session. There is no limit on the audience size. Along with the typical charts and tables for responses, they also offer ‘sticky notes’ that can be moved around the screen. mQlicker also has a wide variety of question/response types and supports rich-text formatting.
Have a look at the live demo of mQlicker on their website.
There are also an extensive range of resources to get you started on their website.
Here’s a demo on using mQlicker