I very interested in this thread; for some time now since dropping Outlook I have been using Google Apps for Business which allows me to add some apps from the marketplace but I am yet to be 100% happy.
I don't really mind the gmail UI and ability to file things; I've even bought an annual sub to ActiveInbox which is a funky little gadget that helps keep clutter out of your inbox. It also offers export to calendar which is a little light on but ok....
Tasks are anther thing that I need to intergrate with email and calendar. Google tasks are as useless as tits on a bull IMO and I've had a go at HiTask, Remember the Milk, Tooldedo and a few iPhone apps that sync but still not real solution. Have been using Chrome with a few gadgets but have also tried Firefox which seems to have a comprehensive extensions list but I haven't found the right mix.
This is probably getting a little of topic Pete but I think for many of us who freelance and have to manage our time to the max a system like this would be neat:
Email - ability to link to calendar and tasks. Export to calendar and automatically set reminders, create tasks for follow-ups to responses that havent replied by a pre-determined time/date; again with reminders. Assign to projects (or folders) relevant to what is appropriate.
Tasks - set to projects, priority, date, and reminders
Projects - set projects with priorities, show assigned tasks, etc
Calendar - ability to switch from diary whether it be daily, weekly or months to see what's on for a particular day to a flow chart that shows what projects are required to be done in order of priority. Ability to drag and drop order of priority if needed.
I know there's a multitude of web based project management tools out there but I can't see the benefit of a monthly subscription which in many cases are quite absurd!!
I stumbled upon Pagico http://www.pagico.com
the other day which requires only a licence purchase and the only thing it is missing it appears is EMAIL!!!!
Anyway, I might try Thunderbird and see what comes of it.