If you are looking for something more robust in terms of productivity solutions (calendar, tasks), then I'd recommend checking out the free trial of
Agendus. It's beyond what I need for day to day organization on my Treo - the built in version has worked fine for me. A good friend of mine and fellow Treo owner swears by it. He was using it on his Tungsten and now on his Treo 650. I've been meaning to install it and give it a try.
According to their description...
In a single application, we've integrated all four of the basic Palm PDA functions--your Calendar, Contacts, Memos and To Dos-- plus an incredibly flexible integrated E-mail and SMS application.
-Chris