Currently I am using Thunderbird with the calendar plugin as my PIM. It has all the basics and is cross platform even though it has none of the other features I want. In addition I can synch it up to scheduleworld so I have a central repository for my contacts and calendars.
Looking at whats out there right now these are the cross platform (web ones are mentioned separately) ones that I have found and why I have ruled them out so far:
1. The apparently defunct Aethera project Clunky to use but solid.
2. The very ambitious Chandler Project Lots of really great analysis of the domain, some great UI ideas, implementation is clunky, slow and not ready for prime time.
3. Citadel. Server side implementation with Web interface. Very complete and solid in many areas.
4. Haystack Seems interesting but I could never keep the semantic concepts from intruding on actually getting work done. I recently went back to the site and was unable to find the Haystack application to see if it had improved.
5. KDE PIM Toolset I have a soft spot in my heart for the KDE Pim tools. Many of them do things well and quickly and have the most common operations eassily accesisble. But again what they are modeling is a less comprehensive model then I am looking for and the metadata model is very limited.
6. NomadPIM This does what it does very well and has all the basics. It even allows tasks to be categorized easily. The use of special views and perspectives is very utilitarian. The base domain model is again a very small subset of what I am looking for.
7. TaskCoach Some great Ideas and UI concepts. Unfortunately it is really only focused on task management.
There are more Web PIMs (inherently multiplatform) then cross platform
standalone applications. Unfortunately I haven't seen any that allow for
disconnected operation. I don't want my schedule to be held hostage by the
vagaries of net connections.
I would recommend that anyone looking at what is current start with: