I am working on a new project that needs a customized (both by design and functionality) e-commerce solution. What do you think are my best open-source choices?
I think what you asking is. I need a cart which is easy to customized and free. Let me put it this way such thing does not exist. I used few of them and they all a mess some less then others but not of them are "easy to use or customized". If you don't mind learning curve and have some patience.
Our experience is that if you want your shopping cart to validate ( a helpful part of search engine optimisation), then you should go for Zen Cart or Magento. OSCommerce is a nightmare and not very flexible.
It's easy to make the cart SE friendly but you can't do anything about the core of application. Now since I worked with Magento for few month I can confirm what a piece of crap. Looks pretty from outside and so ugly inside. Not to mention how slooooooooooow is it. Between Magento and Zencart I will prefer Zencart. Also I'm not big fan of Zencart either.