We recently launched The Power Supply Shop, an e-commerce store built using a combination of ProcessWire and SnipCart.
The site has in excess of 120,000 products and variations, making heavy use of page references as well as SnipCart's "any page can be a product" approach. The site pulls in its data from an external MS SQL database several times a day.
At a glance, the site uses:
ProCache - as well as WireCache for some heavy product listing pages (50k+)
@adrian's Tracy Debugger
A modified version of @Soma's Ajax Search
@mtwebit's fantastic Tasker and DataSet modules.
And that's about it on the module front. For other libraries we're only really using FancyBox.js for product galleries and Anchorific.js for guide pages.
At present the site is geared towards the UK, but if and when this changes I'm looking forward to delving into multi-languages with ProcessWire, something I haven't really worked with yet!