I’ve been using Toggl since early 2016 on a near daily basis. It’s helped me in my day job but also in freelance endeavors and other side projects.
Its web interface is intuitive enough to stay out of your way but also powerful enough to provide useful reports. The mobile app is equally useful, though most of the time I just use their web interface while I’m working anyway.
It’s so integrated into my routine that I don’t really think about it most of the time, I just use it.