If you're a photographer who posts images on the web, check out the Mac app Watermark FX (US$4.99). It lets you add copyright symbols, logos or any text to your photos, one at a time or in batches.
In addition to watermarks, the app also supports waterlines, which will keep your pictures from being used by others. You can also use Watermark FX to batch rename, change size and file type to a group of photos.
You can import PNG, JPEG, GOF, TIFF, PSD and ICNS formats, while outputting PNG, JPEG, GIF and TIFF.
If you deal with a lot of photos, this app can be a timesaver. I tried it on several batches of photos and the app worked as expected, and quickly. The way the app works is very clear, and I don't think users will need a help file.
One thing I did notice is that the image file sizes changed when they were output. A 2.5 MB JPEG became 2 MB JPEG when I added some text. Even more curious, some files increased in size. I could not see any visual change when comparing the two versions, other than the added text, of course. It should also be noted that the camera throws away EXIF data. To some, that will be a feature. Others will want it preserved. By the way, if you want to add a © symbol to your photos, it's 'Option-G' on the Mac keyboard.
Watermark FX is a helpful app. It requires OS X 10.6 or later and a 64-bit processor. I'd also suggest interested photographers take a look at Watermarker for $7.99. It has similar features but does preserve the EXIF data in output files.