It depends very much on what you are trying to accomplish. There are at least 3 broad categories, which I'll try and give some examples. Note that my examples are probably US centric, but the services might also have foreign affiliates, I haven't looked into them all, so...

Selling Prints to Clients

Okay, so you've done a photo shoot with some clients, and they want to order prints from you. Where do you direct them? This will depend somewhat on what country you are in, but here's a few.

• Smugmug
• Pictage
• Shutterfly

Selling prints to strangers

So, you took an awesome landscape picture, and now you want to sell a print to a complete stranger. You could use the above sites, but you might have more luck with a dedicated site. The truth be told, however, is that unless you already have a fairly well known name, you're probably not going to have much luck online from this type of picture. I don't have much experience here, but I'll post a few that I've heard of.

• Cafe Press
• Photo Shelter
• Image Kind

Selling the rights to use your images

This is also known as stock photography. These photos typically either sell a concept, have a type of photo illustrated, but can include landscapes and other general interest pictures. There are at least 2 sub-categories here, I'm more familiar with the micro-stock site. Specifically, you can look into sites such as those listed below.

• Bigstockphoto
• Fotolia
• Dreamstime
• Shutterstock