I couldn't manage without all of these:
Acuity Scheduling - a miracle tool that converts between time zones, allows me to find out lots about people before getting on a call with them, could take payments if I wanted it to, sends out reminders before calls and follow-ups afterwards...
Google Calendar (& G-Suite) - how I organise my time (and it syncs with Acuity, to make sure I don't double-book) and how I share files with clients.
Airstory - I've recently started using this for writing and I really like it (it may take over from Google Drive for sharing files with clients)
Zoom - how I do most of my video calls - I prefer it to Skype because it's easier to record and I find the sound/picture quality more stable
17Hats - I use this to send electronic invoices and contracts, accept payments online, and keep all my client notes together in the right place.
Squarespace - easy to maintain website, with no worries about plug-ins not working
Trello - project management - good for seeing the big picture and the small details
Evernote - really useful web clipper (like making a digital scrapbook)