Retrospectives tend to follow the same format whereever I go and involve alot of post-it notes.
I was in one today where https://www.mentimeter.com/ was used and its definately a site I wish to use in the future. Essentially for free you can setup a single question and allow participants to answer the question via their phones, laptops or whatever by going to a URL and typing in a 6 digit number.
A screen can then be used to show the result in different formats, for instance I saw the answers appearing as a word cloud for one question and as post-it notes appearing on a board for another.
Highly recommend this to bring something different to your meetings especially if wall space is limited.