Where can I find alternatives to...?

LinuxAlt - The Linux Alternative Project

From the website:

My goal is to provide an informational website available to all linux users. The website is currently in beta form and I will periodically update the database with Windows software and the Linux equivalents and alternatives.

The site has a very lengthy list of Windows programs and their Linux alternatives. The website is apparently still under active development.

osalt - open source as alternative

From website:

Our mission is to provide easy access to high quality open source alternatives to well-known commercial products. And remember that open source software is also a freeware alternative.

Just like the other sites suggested it gives alternatives to proprietary programs.

The Linux App Finder's Alternatives page

This website boasts a huge list of software and a search feature. Plus any registered user can add their own alternatives.

alternativeTo

From website:

AlternativeTo is a new approach to finding good software. Tell us what application you want to replace and we give you great alternatives, based on user recommendations

This site has a sizable list of alternatives for a sizable number of programs.