Analyse-It
   Statistical software integrated into Microsoft Excel for linear regression, anova, chi-square and other functions. Download evaluation.
   http://www.analyse-it.com/

EZAnalyze
   Free Excel Add In designed to permit the calculation of basic descriptive and inferential statistics without selecting ranges or typing formulas
   http://www.ezanalyze.com

Microsoft Data Analysis Toolpak
   Free add in included with Excel but not enabled by default. Calculates many descriptive and inferential statistics.
   http://office.microsoft.com/en-us/assistance/HP052038731033.aspx

QETools
   Six Sigma statistical tools (Excel Add-in) from basic descriptive stats and hypothesis testing to control charting, regression, ANOVA and more. Demo download. Purchase online.
   http://www.qetools.com

SigmaXL
   Pareto Pro is an add-in for Microsoft Excel 97, 2000 and XP, which allows you to create single and multiple charts. Performs analysis that will slice and dice data. Includes free demos and product list.
   http://www.sigmaxl.com/

StatTools
   Advanced statistical analysis add-in for Microsoft Excel. StatTools replaces Excel's stats function with 36 robust statistical procedures.
   http://www.palisade.com/stattools/

Unistat Ltd.
   Makes and sells Unistat for Windows: comprehensive stand-alone statistical package that can also be an Excel add-in. A one stop solution for data handling, analysis, and presentation quality scientific graphics.
   http://www.unistat.com/

WinSTAT
   An Excel Add-In providing statistical tests and diagrams. Demonstration download.
   http://www.winstat.com/

XLMiner
   XLMiner is a comprehensive data mining add-in for Excel, with neural nets, classification and regression trees, logistic regression, linear regression, clustering, principal components, and more. Trial version available, requires Excel 2000 or later
   http://www.resample.com/xlminer/index.shtml

XLSTAT
   Data analysis and statistics add-in for Excel. Includes over 40 statistical and data analysis features for statistics experts, researchers, students, and teachers. Buy on-line or download trial version.
   http://www.xlstat.com/

Zumastat
   Provides ability to calculate confidence intervals for correlations, power analysis, and meta analysis (and many more) using summary statistics. Contains many advanced functions. Evaluation download not available, purchase online.
   http://www.zumastat.com

statistiXL
   Statistics add-in for Microsoft Excel. Includes univariate and multivariate tests such as ANOVA, Correlation, Regression, PCA, Factor and Discriminant Analysis.
   http://www.statistixl.com