LR/Mogrify is a plugin for Lightroom 1.3 that can embellish your images as they are exported from Lightroom. It's flexible interface allows you to add borders, watermarks and text annotations to your images.
Please note that users of Lightroom 2 or higher would be much better served by the sequel : LR/Mogrify 2.
LR/Mogrify uses ImageMagick to process the images. ImageMagick is a collection of powerful, freeware command line utilities for processing images.
LR/Mogrify and LR2/Mogrify are both donationware plugins and are distributed via the Photograper's Toolbox. The trial version limits the number of images that can be exported in one go to ten. Donating towards one of the projects removes this restriction and will give you a serial number that works with both.