Sli.do bases the presentations on events, and each event under the free plan can have 3 polls, and up to 1,000 participants. But, moving past the polls, participants can ask questions and those questions can be voted on by other participants. Kind of like how Google Moderator worked. In beta is a live slides feature, which would allow the participants to follow along the presentation with their device.
I could see using Sli.do when you want to mix up your presentations. And the ability to collect questions from the audience would be beneficial when presenting to larger groups of people.