Impression for Aperture

Adds high-quality watermarks to your photos.
The original RAW images are not altered.
Multiple photos can be watermarked with one click.
Existing PSD (Photoshop) or PNG images can be used as watermarks.
Watermarks can also be created from a RTF text file. The font and color from the file is used for generating the watermark.
The Impression plugin can place watermarks horizontally and vertically.
Controls are available for adjusting the transparency, position, margin and size.
It beats Aperture's built-in watermarks.