A UserExperienceMasterpiece is a real-world example of the best possible kind of user experience that a person can have when using software to perform a task. Probably implies a well-thought-out layout for user interface widgets. Exceptionally clever back-end techniques probably help by automating tedious tasks and speeding up complicated operations. Note that a UserExperienceMasterpiece is not necessarily implemented as a CodeMasterpiece. However, an application clearly must satisfy a base level of robustness to be a UserExperienceMasterpiece, as nobody seems to like software crashes or data loss.
The definition is necessarily subjective; it would be nice to see individual thoughts on the matter, as well as some consensus on what software is generally considered to be a masterpiece.
Some candidates, collected from the SoftwareMasterpiece page and seen in various places over the web and in print publications: