The other day we launched the Google Base Store Connector.
The progression to the final version was pretty neat. When I started working on the project there was a functional demo that had already been built, wizard-style. I came up with a few user scenarios that led to functions like storing account information so users can easily update their items in one click. That is, if someone has an Ebay store and they want to get listed on Google, it'll be better for them if they can store account information so all they have to do is periodically click something to say "okay, update my items" rather than having to re-enter account information each time.
(Yes, there would be faster and more humane ways to do this that don't involve an intermediary application, but those are outside of technical scope.)
The engineers and PMs on the project did an awesome job getting the details right. For example, based on the store you select (eBay versus Amazon, for example) the textual labels change next to the input fields. We used the language from each of the source sites: Amazon uses the terminology "sign in" with your "email address" instead of something like "log in" with your "username" or "user id". Yahoo! has a "Yahoo! ID". This keeps it just a little bit more familar for users. They probably couldn't tell you if Yahoo! calls it an "id", a "user id", or your "yahoo signin", but seeing the same verbiage will trigger subconscious memory and feel a little more "right". Cognitive priming, anyone?
So: a very small piece of software, but reviews on Ebay's forums generally commented on how simple it was to use. Yay! We wouldn't have gotten there without a comittment to good technology from the PMs and engineers. Good user experience is only partly UI design; a lot has to do with the engineering behind the interface.