Some good free digital watermark programs are uMark, Star Watermark and TSR Watermark Image. UMark supports watermarks of various formats, including text, image, QR code and shape. It can watermark up to 50 images at a time. It is only available for Windows.
Star Watermark is available for Windows and Mac OSX. It supports text and image watermarks. It lets users create watermark templates and apply a template to images for batch watermarking.
TSR Watermark Image can batch watermark thousands of photos at a time. Its One Click Share feature lets users share their watermarked photos on Facebook or upload to WordPress and FTP sites.