I've been wresting with my own Personal Information Management issues for years. I work on a number of different computers on several different platforms, some of which (like those at Uni) I don't have admin access to.

For a long time I've used Kontact, which contains the KDE applications KMail, Korganiser, Akregator and Knode for my mail, calendar, RSS and News access. For a few years I leveraged the then somewhat unique ability of Kontact to store my address book, calendar and task list as iCal format in an IMAP folder. There are a few problems with this, firstly I need a system i can install KDE on, or one already running it, so one can effectively rule windows out. Secondly there are some stability issues, I'd hoped they would have been fixed with the arrival of KDE4, but so far the companion PIM packages haven't been completed.

I am quite fond of Thunderbird, it gives me just enough control and flexibility tonot be annoying, yet doesn't overload me with stuff I'll never use. Whilst the future of Thunderbird is uncertain, thanks to it's open source development and the use of open standards, I feel I have a strong chance of not ending up in a developmental dead end.

Requirements

• IMAP email account (easy enough to get hold of, I use 1and1 instant mail, googlemail and my own mailserver).
• Thunderbird an Open Source mail client visually similar to Outlook Express, sister development to Firefox
  
• Lightning an add-on for Thunderbird that addscalendar functionality, one of the fewcalendar applciations that can synchronise bi-directionally with Google Calendar.
• Sync Kolab, an add-on for Thunderbird that actually performs the synchronisation with your IMAP folders.

Installing the add-ons in Thunderbird is trivially easy, and instruction are available on the add-on pages I've linked to so I won't go into it here. Then you need to make some new folders for your Calendar, Contacts and Tasks, and go to Tools -> SyncKolab options to get a wizzard that will talk you through the process.

Results

I now have my address book, calendar and todo list available on every machine I have installed the above software onto, and I can change or add data on any computer, and have those changes visible on all the ones I use. I've been using this setup for about a week now and haven't had any annoyances.

If for some reason Thunderbird becomes unuseable, there are a number of alternative clients.

Further Information 

http://www.gargan.org/extensions/synckolab.html 

http://kolab.org/

http://wiki.kolab.org/

http://www.kontact.org/