what do you mean by 'aggregate'? Do you want to aggregate row or column wise? Can you elaborate a bit more on what you intend to do?

The Java-Snippet node can only help when you aggregate row-wise, for instance if you want to append a new column to the table containing the sum of the i-th and j-th cell of a each row.

Column-wise aggregation is not as easy. Have you had a look at the Jython nodes http://www.knime.org/download_extensions.html#python? They should allow you to apply much more complex transformation. Not that I have used them often but they are definitely powerful (thanks to Tripos for contributing them).

there is the "Statistics View" node (category "Statistics" in the node repository), that computes the Min, Max, Mean, Std. Deviation and Variance for each column/attribute. It doesn't show the sum of all values in one column though, if that is what you needed.