Microsoft Excel

OK ... what about Microsoft Excel (or similar spreadsheet applications)? Should it be included on a list of statistical software packages?

This is debatable.

Certainly Excel (and other spreadsheet applications) includes some functionality for statistical methods, and there are several packages that are add-ons that are available commercially. (See QI Macros)
Having admitted this, I will also note that it is very easy to mis-use the statistical functionality offered in a spreadsheet package ... easier, perhaps, then it is to mis-use functionality offered in a statistical software package. I have personally witnessed a Ph.D. (in a physical science) inappropriately use the formula for calculating the standard deviation of a population when the appropriate function would have been the standard deviation of a sample. This is only one of many examples of misuse due to misunderstanding of the tool and its limitations. When using more advanced methods (regression, ANOVA), it becomes even easier to mis-use a spreadsheet application.
I have noticed that those who typically use spreadsheet applications for statistical analysis usually fail to perform and report diagnostic tests to check the assumptions of their model and its statistical method(s).
There are also computational issues that a user of any such application needs to be aware of. The articles below discuss many of these issues.

Included below are various links to articles that discuss this topic.

Paper on "Spreadsheet Addiction" by Patrick Burns that nicely outlines many of the pitfalls of using spreadsheets in applications perhaps better suited to other types of software packages.
R-help list for R-GUI forum (thread on the use of spreadsheets)
Notes from Michael Dewey on using Microsoft Excel for statistical analysis (see his comments on Open Office at "Free Software for Statistics").
Comments from http://www.statisticalengineering.com/Weibull/excel.html
Research regarding spreadsheet errors etc http://panko.shidler.hawaii.edu/ssr/Mypapers/whatknow.htm
Paper by IBM - SPSS (attached at the bottom of this page; also here) entitled "The Risks of Using Spreadsheets for Statistical Analysis"
"Why R and not spreadsheets?" (http://www.burns-stat.com/first-step-towards-r-spreadsheets/)
"Living it up with computational errors." (http://www.burns-stat.com/living-it-up-with-computational-errors/)
Article from Financial Times (http://is.gd/jzc0xl) "The Siren Call of Microsoft Excel"
Why R is Better Than Excel for Fantasy Football (and most other) Data Analysis (http://is.gd/f4g07D)

An interesting discussion thread with several useful references is available in a LinkedIn Group.

GROUP: The R Project for Statistical Computing
DISCUSSION: "Excel is a wonderful tool for many things. Statistics is not among them." started by Valentina Cupac

Interesting Excel add-ins: