I look on flickr.com to get ideas. I think you can search "watermarks" there. Or just look around at everyone's photos. iPhoto doesn't have one, but APerture does and also Lightroom. I like Lightroom because you can use an image or text.
Before using LR, I used iPhoto a lot and I found an online watermarking program. It was free but idk how safe or effective it was. If you are serious about photography, invest in software. Either Lightroom 3 or Aperture 3 would get you in that direction. If you have a friend that has Photoshop, ask them to create you a watermark.