I have used phreebooks (now a part of a larger package called phreedom) for a while now. It's amazing and flexible. There is a zen cart connector that allows you to download orders from zc to pb, upload products from pb to zc and a sync to update inventory and other product data.
I am working with the developer to augment the zc connector to completely map zc product attributes to specific skus in pb. You can either append a defined string to the model number (which is an sku in pb) or specify a different pb sku per attribute. So, for instance if you are selling t shirts and the base sku is tshirt1 and you have different sizes as attributes in zc you can append a sm or md or whatever so the sku becomes tshirt1sm. Let's say a t shirt that is red has a different sku, so you could tell it to make a red tshirt1 have an sku of redshirt. You will also be able to specify quantities of attributes. So if I sell a widget and the attribute is screws to mount it to a wall, and widget 1 uses 2 screws(a different sku than widget 1) and widget 2 uses 3 screws(the same screw sku as in widget 1) you can specify this, and in pb a line item is created in the sales order with the quantity and sku. All of this is set up in attributes controller in zc. I have yet to find a way to accurately import attributes in any other accounting/erp software including quickbooks 2011 premier.
Another great thing about pb is the author is active and available on the forum, he is open minded and takes people's suggestions seriously and with respect. He also helps solve problems specific to a user's needs, posting code and different reports as needed.
I wholeheartedly endorse it and suggest that everyone using zc check it out. It's entirely adaptable to any country's bookkeeping regulations so can be used by any around the world.
Have any more questions? Just ask, I will help if I can. I will be adding the zc connector to the downloads section here in a few weeks once we finish ironing out the attributes mapping.