Alternativeto.net is a very interesting online application. It shows alternatives to similar applications:
AlternativeTo is a new approach to finding good software. Tell us what application you want to replace and we give you suggestions on great alternatives! Instead of listing thousands of more or less crappy applications in a category, we make each application into a category.
Application alternatives are often open source or freeware applications.
Take Notepad++ as an example. The top alternative choice is IBM Eclipse which is of course a totally different application for editing because it serves another purpose. Similar applications like notepad2 are mentioned, or not at all as e.g. Textpad. The reason is that the platform does not sort the applications by similarity but by popularity votes. Features like application tagging could create a semantic network of available software.