It's often useful to place some text (such as 'DRAFT') in the background of every page of a document. For LaTeX users, this can be achieved with the draftcopy package. This can deal with many types of DVI processors (in the same way that the graphics package does) and knows translations for the word 'DRAFT' into a wide range of languages (though you can choose your own word, too).
More elaborate watermarks may be achieved using the eso-pic package, which in turn uses the package everyshi, both part of Martin Schröder's ms bundle.