Slack has a pretty nice integration with appear.in. It's a free in-browser video conferencing service. We've used it with Slack very effectively in the past!
We are an entirely remote company, and we found that videochat during our standups really helps. Currently we are paying $5 a month per user for Google apps and only using a very small portion of it (basically just the video chatting), and they allow up to 25 people which is perfect for us.
we routinely have meetings with 15-30 people and we use Sococo[1] for all our audio and video. The office layout is also really nice. Everything else we've tried for video hasn't cut it, in comparison.
Note that Sococo will try to push you to their new browser based product. Last time we tried it, it was far worse than the 'classic' version we still use. But they don't really show 'classic' on their website anymore.
That sounds horrible.
You get the occasional feedback screech or something, but for the most part we aren't animals and we all know how to mute our mic's. And the added benefit of a face means you can get more body-language which helps prevent the "talking over" that happens on conference calls.
Plus the way we have it setup now we can all "mute" one another, so if someone is causing an issue and doesn't know it (or stepped away for a second) we can instantly mute them and move on.