Since I'm a developer I have to have Visual Studio. That's essential for my job. Then because I do database development work I've got to have SSMS. We do reports and are trying to migrate away from Crystal, so I've also got to have SSRS. But because we're still working with Crystal ("curses , Batman!") I've got to have that installed as well. MS Office is also necessary for me. Some FTP client (I'm not particular, just whatever is convenient). I also like Paint.NET for doing graphics work. And I also use Blend for Visual Studio for doing XAML design. I need Fiddler 2 for troubleshooting HTTP traffic with our web apps. And I love Notepad++ installed as well.

Those are what I need to be productive.