Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
While it's always been an exemplary mail client, Mozilla Thunderbird has always desperately lacked one vital component - an integrated calendar. Despite users baying for this one feature, the Mozilla team stuck to its guns a little too long, wanting to deliver an application that did mail and nothing else.
Personally, I believe that this was a tactical error from the heroes of open source. A calendar in a mail application just makes sense - it's the one application that gets the most attention on most people's desktops; the application that you rely on for your most personal communication, business and fun. The calendar component made sense to Microsoft Outlook, Evolution and KDE's awesome Kontact suite, so why not Thunderbird?
But finally there is hope for us ardent Thunderbird supporters. It's called Lightning, a new-born integrated calendar currently in version 0.1.
They're not kidding when they say version 0.1. Lightning is seriously buggy, and lacks the intuitive interface of its competitors. Currently, my Lightning is pretty convinced that it's still yesterday. Or perhaps Friday. It's not really sure, but it's definitely not Monday. (If only it were right.)