Simple task/project management tool for freelancers?

I'm a freelance consultant & developer working for approx. 30-40 clients since many years. Typically, 2-3 "bigger active projects" run in parallel to spontaneously occurring tasks (small or very urgent).

I found excellent solutions for communication, time tracking, invoicing, etc. but still search for a simple but effective solution for task / project management, especially with a focus on resource (my time) planning.

For me, project management tools target teams and feel – at least for my requirements – too extensive and complex. Pure task managers lack in resource planning features.

Does anyone know or even use a system fulfilling similar requirements, especially suitable for freelancers without a team?