Beautiful software is one that does things that you don’t expect and things that are actually useful for you in some way (e.g. that reduce the effort for the task that you have in mind).
I can think of at least three examples of such software:
- Mozilla Firefox – It’s plugin-based architecture and its capacity to update, etc. makes it a great piece of software. The tools that the community around Firefox created (XUL Runner, XPCOM, etc.) make an excellent base to develop other great desktop applications.
- Gcc – the GNU compiler collection is a hardcore piece of software. Maybe one of the most complex pieces of software available today as open-source and with such brilliant minds working on it.
- WordPress – this open-source blog engine is a wonderful web application. It also has a architecure based on core+plugins and has capacity to update, themes, etc. It makes an excellente base to develop other web-based applications.