First, start out by clicking on your location. CraigsList.com has USA and International options. Once you're at your selected location, you'll find a variety of categories. Click around and see what's available. From my own research, I know most postings for photographers end up under the "services" section, and the category "creative".
Find the services section, and click on the "creative" link. This will take you to postings made under this category.
Browse through some of the postings, and see what people are writing. As I'm writing this, there are offers for web design, writing and editing, and photography.
Keep in mind that anyone can post. You can create a post selling your photography services. Or you can create a post asking for bids on services.
Let me show you how it can work. A friend recently needed a business portrait, and posted a "Wanted" ad on CraigsList.com. Within one day, she had several contacts offering photography services. She chose one and had a professional portrait created near her office. She was very happy with the results.
This is a very simple system, yet the potential is amazing. CraigsList.com is the 7th most popular site online. With all of that traffic, shouldn't you be giving it a try?