Many people have wanted the ability to ditch Microsoft Outlook and replace it with Mozilla Thunderbird. Your solution is here and it is called Mozilla Lightning. This is nice because it will let you keep track of your everyday schedule right in Thunderbird. This is currently version 0.1RC1 so it has not been officially released and make sure to read the release notes so that you understand the technicalities before installing and using it (like the lack of Undo/Redo). I don’t know how well this will take off once the Google Calendar gets rolling.