OKDate is an open source date library I just released for objective-c. If you've ever worked with NSDate in Objective-C you know how painful it can be. Doing simple tasks like getting the year or adding a month are excruciatingly verbose tasks. OKDate takes the approach of subclassing NSDate (as opposed to using categories) which allows it to change the underlying implementation of NSDate.
With OKDate, dates are mutable – meaning you can change them by setting or adding things like hours or months. To top it off, there are literally hundreds of convenience methods for creating, parsing, rendering, and otherwise using dates.
As you might know, I'm not an obstinate supporter of Test Driven Development. Quite the contrary in many cases. But in my opinion this library fit the bill perfectly, so it was developed test-first.