What I’ve Done
Most audiences like a bit of friendly competition. Enter Kahoot - a web-based tool provides a quiz style interface, where participants use thier phones as game pads to key in answers to projected questions. Respondents are rewarded with points based on both accuracy and speed. It's fun, fast-paced, and easy to set up.
Kahoot allows me to either take a quick survey of gaps in knowledge, or to solidify it after the fact. I often style these games a "learning trivia" experience.
I like Poll Everywhere to take the temperature in the room, as a discussion prompt, or to build a word wall.
I have been also using Google Slides Q&A functionality. What I find interesting is the ability for the attendee to queue up thier questions as they happen, in a virtual parking lot. It also allows the facilitator to easily download results, keeping a record of questions.