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

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/

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/

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

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/

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