List of Data Mining / Forcasting Competitions

Kaggle
http://www.kaggle.com/Competitions

TunedIT Solutions
http://tunedit.org/challenges/

Causality Workbench (none current as of Aug 2011)
http://www.causality.inf.ethz.ch/home.php

DARPA's Shredder Challenge (document reconstruction)
Closes December 5, 2011
http://www.shredderchallenge.com/

1st International Competition of Time Series Forecasting
Closes 10th of January 2012
http://www.caos.inf.uc3m.es/~jperalta/ICTSF/

if you know of any more sites offering data mining / forecasting / machine learning competitions please leave a comment!  Thanks.