Slack has been great. As our company grows, we have had to train ourselves on how to maintain it. It works perfectly for quick back-and-forths. With it, we can communicate without filling our email inbox.
We user FreeConference for conference calls. Having the video option makes it a lot easier to communicate. We tried Skype but we felt that we needed a more professional tool.
We use Calendly to arrange meetings with prospective clients.
Google Calendar is easy to use and connects to everything. It works very well to arrange meetings internationally.
Other people are using tools that are more specific to engineering but we find that Trello works perfectly for us. Trello is a very good project management system--it’s adaptable and let us keep things organized. We have been trying to learn more and more about it to exploit its full potential.