The analysis toolkit beefs up EXCEL's statistical capabilities allowing for example the analysis of one and two-factor analysis of variance. It is not usually in the EXCEL menu by default and so needs to be added in using the add-ins option under the tools menu. If successful the Data Analysis option will be available at the bottom of the tools menu. Details of how to install the toolkit are given here.
A free to download add-on created by Charles Zaiontz is available from here which extends the range of statistical methods that can be performed in EXCEL (covers version 2007+).
There is also free downloadable add-on software called RExcel which allows R routines to be used within EXCEL. This is available from here. Note: Rcmdr gives a very similar gui interface to RExcel, using tcl scripts, in R (rather than in Excel) which will allow the reading in of Excel data files (and those in formats from other software such as SPSS) into R.