Mozilla Thunderbird, the only program that I keep running during my whole work day, has just been upgraded to version 2. Congratulations, Scott and the Thunderbird team!
I encourage anyone looking for an excellent mail client to replace e.g. Outlook or Outlook Express to try Mozilla Thunderbird. If you want to be convinced, read the features page.
At my workplace, I’m still the only one using it because everyone else is a bit narrow-minded and wants everything to be just the way they’re used to (which in this case means Microsoft Office). However, I’m slowly starting to convince people to look for open source alternatives. Mozilla Firefox is already used by circa five persons and counting, and, even to my own surprise, the GIMP has replaced Photoshop for one of my colleagues.