I'll lay my cards on the table here: I've recently discovered I have ADD (attention deficit disorder) and this generally manifests itself in a tendancy to procrastinate and i'm easily distracted. So, as a Digital Agency owner, the ability to manage multiple tasks and projects requires really well designed tools that can make it not just easier but enjoyable. The good news is they exist. There are a number of free platforms that can work together if you're really on a budget but to get the best solution it's worth paying a few £ a week as you'll save it through greater productivity.
Let's start by considering the tasks that have to be juggled as a small business owner. The first and most obvious is project/task management. If you're working on multiple projects, then there is a good chance you're working with other team-members or freelancers so the ablity to communicate with them in an environment where you can keep track of all the seperate discussions is paramount. Secondly you need to be able to manage/oversee the projects. Set goals, timelines and assign tasks to people.
Lastly, and very specifically for business owners, lets consider the act of acquiring new business. Call it a sales pipeline if you must but a customer relationship management (CRM) element is essential to give you an overview of enquiries and where they are in the sales process.
Trello
Trello is a great tool for freelancers and small business owners for managing tasks and actions. It's a very visual tool with 'cards' being laid out within 'boards' rather like a T card job board. As with all of the platforms cited in this article, they work across all platforms and devices and usually have native apps for iOS and Android. If you like this way of laying out data, it's known as Kanban.

Asana

Hubspot
Hubspot is a multifaceted tool for sales, CRM, marketing and operations. It's very powerful and you can spend £1000's if you go for all the options but at the heart of it is a free CRM that is extremely good. I've used it for years and will integrate with many other online (SaaS) platforms. The other nice feature with the free version is the sales pipeline tool that is very easy to use and looks great too.

Clickup
ClickUp is an all in one tool that encompasses everything that i've already talked about. It can manage tasks, projects, timelines, CRM, sales pipeline, the lot. There is another very similar platform called Monday.com that offers more or less the same offering. I would not say that one is better than the other, but personally I prefer the aesthetic and functionality of Clickup so that is what my agency uses.

Other communications tools to consider
Slack
One of a number of tools that are designed for collaboration with remote colleagues. Easy to use and intuitive. Text, chat, audio and video. And free too though paid versions are available. Recommended.
Microsoft Teams
This is Microsoft's take on group collaboration but to be honest it's frankly baffling and to be avoided unless you're a dyed in the wool microsoft fan.
Discord Server
Like the idea of managing your own community away from the prying eyes of our digital overlords ? Then set up your own Discord Server. This evolved from the gaming community but offers so much more. Recommended.