Weather predictions would be much less reliable if the models that compute them were not corrected by observations. Such use of observations to enhance the accuracy of computer forecasts is called data-assimilation. It is very powerful, but until now also rather expensive. The new OpenDA (Open Data-Assimilation) software brings data-assimilation into the reach of a far greater number of users by allowing it to be implemented for any computer model without the need for a large investment. For certain applications, this may improve the accuracy of model predictions by more than a factor of two. But OpenDA is also useful in fields that already use data-assimilation as it greatly facilitates the adoption of new techniques and improved implementations of algorithms. The OpenDA software has been developed by Delft University of Technology, research centre Deltares and the scientific engineering firm VORtech, all in the Netherlands. As of 10 May, 2010, it will be available as Open Source.