You can go with one of the big guys, like EBay, Yahoo! Store or Amazon WebStore. These options give you a certain amount of credibility and traffic. As you might expect the take a cut of the profit through a monthly fee or a percentage of sales or both. In return for this they take care of the merchant account and a certain amount of the marketing. If you are in the right market, these options can work.
Another option is some of the middle-tier SAAS e-commerce sites out there. Some of the prominent ones are Shopify and Vcart. These services will generally cost less (when considering per sale fee) then EBay, Yahoo! Store or Amazon WebStore. While the cost is lower, the marketing that the bigger guys help with either goes away entirely or is greatly reduced. These can be useful if you want to do a lot of marketing for website but don't have the technical expertise (or desire) to manage a custom built store.
The final option out there is to use an open-source commerce solution. You can go two ways here. A pure commerce solution like Miva Merchant, osCommerce, Zen Cart, and Ubercart. Or, you can use an extension of an open source CMS solution out there like Joomla or Drupal. While have experience with most of these options here, I prefer to go with a pure commerce solution because that is there main focus while the CMS extension is just an afterthought.
I will go more in depth with discussion about some of these options as I use them. Currently I am developing under an osCommerce platform, Joomla and Drupal extensions. While osCommerce is a little rough around the edges, if you put the time in, it seems to be the best of the ones I've tried. I still intend to try out Miva Merchant, Zen Cart and Ubercart so I can have a broader view of the options out there. As I try these out I'll have some more posts here.