In my job as a professional science writer, I use no less than eight different software packages to get the job done. And that number is tending to go up, not down.
On the writing side, I use word processor, obviously. I'm using Microsoft Word for most of my work, but lately I've started using Google Docs when I have to collaborate with students.
A word processor alone doesn't help much with the task of references, though, so software package number 2 is a specialized reference manager; I'm currently using EndNote.
Now that I have the words under control, I need to deal with numbers. Microsoft Excel is great for data entry and simple manipulation, but again, I'm moving towards Google Docs for certain tasks. But Excel can't do the sort of high end analysis that I need as a scientist. So I have a statistics package (currently SPSS), and scientific graphing software (currently Origin), bringing the total to five.
I find I also need two kinds of graphics software: one for vector based images, and one for a pixel based images. I've been using Corel Draw since version 3, so I'm going to continue to use it for as long as I can for the vector images. Luckily, the package also includes Corel Photo-Paint for the pixel images.
I could, strictly speaking, stop there. But I consider posters a sort of manuscript, and I could use my graphics packages for making posters -- and have done so in the past. But I've found a simple desktop publishing program can actually be faster and easier. Microsoft Publisher is little known but very useful in this regard.
And that's what I use just for mastering my manuscripts, never mind presentations or websites or other odds and ends.