PhotoBulk Lite is a stripped down version of Eltima Software's PhotoBulk 2 which allows you to create your own unique watermark with a variety of settings: choose Font, Style, adjust the Opacity and the Color of the text. Also, you can control angle and position of your logo in the live-preview box.
This app can be especially handy if all you want to do is add "PROOF" to the middle of your photo previews. You can download PhotoBulk Lite on the Apple Mac App Store.
For more advanced watermarking, PhotoBulk 2 is likely worth its $9.99 price.