When you use some application, you may always think about alternatives. Whether its web browsers or desktop applications, you may like to check which are the applications available that provides the similar or more functionalities. Searching through Google or Yahoo! may not always provides your the alternative applications. However AlternativeTo.net do that.
AlternativeTo is a new approach to finding good software. You can search for any application that you want to replace and it will give you suggestions on great alternatives. Instead of listing thousands of more or less crappy applications in a category, it makes each application into a category. Think of it like forever evolving blog posts about good alternatives to the software that you're not satisfied with. And the "blog posts" are generated by you through suggestions, comments and votes.
You can also search an application with respect to OS (windows, Linux, Mac …).