an e-commerce system around FoxyCart. There is a monthly charge of $19/mo, but it's a flat rate and independent of how many things you sell and how much money you make.
I've never worked with Shopify myself, but know several developers who have setup their clients on it. I've only heard good things and I trust their judgement. If I were to launch a store and needed it up asap, I'd probably look at Shopify first, then compare other e-commerce solutions against it.