TubeBuddy - a great way to optimize your videos so you can be found by the people who want to see your videos and grow your channel. The free plugin works great, and if you like, you can get one of the paid versions as you grow. Even the free extension is incredibly useful!
17hats - a client management system that makes billing and followups simple and allows for a lot of automation. Reasonably priced and very helpful in a small biz situation.
Acuity Scheduling - allows clients to self-schedule appointments and to pay their invoices all at once. Makes life SO much easier for all involved and pays for itself with the first appointment.