Here is some good links for some public datasets for testing data mining algorithms
All the datasets of the previous SIGKDD KDD-CUP
. Some of them are very large!
The UCI KDD Archive
that contains around 40 larges datasets of different types.
The UCI Machine Learning repository
offers 185 public datasets.
The Frequent Itemsets Mining Datasets Repository
provides many classical datasets for frequent itemset mining. It also includes several implementations of classical algorithm such as Apriori and others for mining frequent itemsets, frequent closed itemsets, maximal itemsets, etc.
A few datasets and implementations of data mining algorithms can also be found on the web page of M. Zaki
A lots of links to free datasets : http://www.kdnuggets.com/datasets/index.html