For most, Adobe Photoshop may be out of budget for occasional or personal use. GIMP can solve that problem. The GNU Image Manipulation Program (GIMP) is a very versatile image and graphic tool that has many image effects and many ways to edit images. The biggest problems I have with it are that it runs in x11 and uses the control key on the keyboard for shortcuts instead of the command key like all other apps do. I believe there are some native Aqua Gimps out there, but the one I tried wouldn’t load up. You can download one of the versions here.
For users that just want a little bit more than iPhoto but don’t need all the geeky features of GIMP, try Photoshop Express (free). It is a web-app that you sign up for in order to edit photos online. You can do some basic effects that iPhoto does not have. There is even a live demo you can try out before you sign up. Check this free web-app out @ Photoshop.com.