Frequently I use the Wisconsin Breast Cancer Dataset for demonstrating the Data Mining Addins for Office - enough people asked, so I made it available as an Excel 2007 file (free login required).  For purists, the original data is available at the Machine Learning repository, which is a great location for many sample datasets.

Here are some screenshots of the data mining add-ins applied to this dataset

Figure 1:  Key Factor Analysis showing differences between benign and malignant tumors

Key factors discriminating malignant and benign tumors

Figure 2: Detect categories showing malignancy across detected groups.  Note two purely malignant categories suggesting differing classes of malignant tumors.

Malignancy across categories detected by Table Analysis Tools

Figure 3: Decision tree to predict diagnosis, with nodes shaded based on likelihood of malignancy.

Diagnosis Decision Tree