GTD

Anydo comes with a very useful daily review and reminder. Anydo also integrates with its own utilities such as calenders.

Not so lightweight but still very good for managing life especially since it has got a lot of integrations from a lot of other services.

Google Keep is light-weight. It comes with powerful search. The App ships with simple drawing and handwriting.

Task management tool backed by Microsoft and works on most platforms.

A mix of todo list and pomodoro timer, with sync across devices and weekly report.