Formative looks to fill that void and in an awesome way. I met Craig Jones of Formative at CUE and was impressed with his product for giving Formative Assessments.
Creating formative assessments is extremely easy and watching students go through the assessments live is awesome. What impressed me the most is not that the product is free, but you can add basically any type of document into formative and create text boxes in that document for students to fill out when taking their assessment. I look forward to making Formative one of my daily go-tos for seeing if my students really did 'get it.'
I am not going to compare Formative and Kahoot because I think they attract a different audience and age-group. I like that Formative gives me more options for the types of documents I can use in creating my assessments. Kahoot is a great game tool and my students can challenge each other. I know only too well how they like to do this when using Quizlet for vocabulary.