While I am not a professional photographer, nor do I try to make a living selling photography at this time, I do hope this list helps people interested in this  arena locate some of the more popular and reputable online services (the ones mentioned are the ones I’ve seen photographers mention the most in a variety of forums and social networks).

Some popular services to sell your photographs online:

DeviantArt
Etsy (more of an arts & crafts community)
Flickr (Flickr largely has partners where you can get work done; most of the photogs I’ve talked to sell their prints on one-off occassions)
Fotomoto
Imagekind
Shutterfly
SmugMug
ZenFolio

A couple of things to keep in mind:

1. While these services will help you display your images, you will largely be responsible for promoting your work
(this will take marketing on your end). I have some quick tips for promoting your photoblog, for example, but this isn’t the only thing you could do and/or should be doing.
2. Some of these services do charge you to host your photos (subscription fees).

3. Some of these services will only charge you if an order is actually placed (varies by company for costs…but you can generally expect somewhere close to 15% of the total sale price).

4. Many of these services, if not all, will handle the fulfillment process for you (shipping, printing, etc.)

5. All of these services may not offer what you need if you are an event or wedding photographer, such as password-protected galleries and the like,  so you might want to look at services like Instaproofs.

Reality check:
1. Many photographers, even serious professionals, will tell you that selling prints online is not an easy
thing to do. As mentioned above in the paragraph above, it takes a fair amount of marketing to get your name and images out there.

2. Selling photographs online should be viewed as a passive form of income for most.

3. Set reasonable prices for your work. Fields like nature and wildlife photography are very saturated with many images from pros and amateurs alike, so  you would really only want to charge a great deal if your images are very unique.

4. You may have to sell across multiple channels to sell images, inlcuding using more than one of the online services mentioned above.

Stock photography:

Since this is not even remotely close to an area of  familiarity or expertise of mine, I am not going to cover this arena at all.  This article on selling photos online with stock agencies should give you some leads.