Picasa will do it. I use picasa 3. You need to double click the picture to get to the editing menu. On the first screen, just to the left of the pic, you'll see a pic with ABC over the image. Click there to get options. You type the text you want watermarked, and set the transparency level where you're happy with it, then save, & upload.
Hope that helps you!
I'm attaching an example I did this way.