There are two sites that I haven't seen mentioned anywhere in this forum yet:
CafePress - Works similarly to something like Redbubble (from what I've seen of Redbubble, anyway). You upload the pictures and select which products you want to offer, select your markup by either percentage or dollar amount, and they do all the printing and shipping for you. The thing I like about Cafe Press is that you can sell more than just prints - you can put your photos on note cards, framed tiles, notebooks, calendars, mugs, mousepads, and all sorts of other products. I've only been using it for a few weeks and no sales yet, but I also haven't pushed the marketing too much or added too many products. You really need to get the premium store to make it worthwhile (otherwise you're only allowed to upload one image) - but the premium store is only $6.95 a month.
My CafePress store can be found here.
Art Garage Sale - This one is for those of you who want to print and ship your work yourselves. The site takes no comissions, there are very inexpensive listing fees to list an item. It's still a fairly new site, but I think it's got a ton of potential as it grows and if you market your "garage" well.
Take a gander at my garage here. Oh, and because they're new, they're running a "no-listing fee" special from now until February, I believe (and a listing is good for three months, so if you list a bunch of stuff now and then relist it in February right before the offer expires, you'll essentially get six months of free trial of the site to build your client base).