Every Reflective Thinker needs one.
Thomas Erickson built an app he called Proteus on HyperCard back in 1996. I use it to maintain a To-Do List, to keep notes on meetings, to compose and send EMail, and as a repository for information (e.g., email messages) on which I want to reflect or act later on. I keep all the information for a day on one page in a long scrolling field.
also a specific piece of software from CircusPonies
see also Google Notebook