Photoshop has the ability to add a watermark, but it's a bit of a pain. You have to create text as a separate layer, determine transparency/blending/etc. Then once you have it down, if you want to do it in the future or create a batch watermark, you have to go into file>automate>batch. It's not hard really, just a bit of a pain. I actually like Picasa better; it is much easier, you just enter the text of your watermark and check the "watermark" box under Google+ photos. This obviously requires that you use Google+ and Picasa, which makes it useless if you do not. 

Another option would be to use a free web based watermarking service. Most of these just add a little rectangular overlay to your image. Functional, but it doesn't necessarily look all that beautiful. You can give a couple of them a try (watermark.ws and mywatermark for starters) and see if it looks good enough for you.