Agenda was written about ten years ago by Mitch Kapor and Jerry Kaplan, the founders of Lotus. They were frustrated with available tools for time management and as a result wrote a piece of software that stands even today as one of the greatest PIMs ever developed.
There is some interesting history about the evolution of Agenda in Kaplan's book, "Startup". Some say that "Agenda" is an equal to "Lotus 123" in terms of importance and I would certainly not argue with that!
Unfortunately Agenda never attained the commercial success of 123 and Lotus eventually put Agenda out to pasture. However, there are thousands of people still using Agenda today for all kinds of complex data management tasks because they cannot find anything comparable from the Windows/UNIX software vendors. A mind-boggling situation, but true!