Even for someone like me, who knows about open source software pretty well, it can be hard to find certain software. And I'm sure it would be harder for other people to find out about good open source software.
At my new job we've been working on a site to do just that: Fossfor.us. FOSS stands for Free Open Source Software, and we actually are making the site for people who don't know what FOSS is at all. There are loads of screenshots, video demos, feature lists, etc to help you know if software is what you want or not. And you can browse and search by function or by alternative (e.g. find me an alternative to AIM, or Microsoft Office).
We've got a lot of features (and some bugs) to still work on, but go check it out and download some awesome free software. You deserve it.
Feel free to post suggestions here, or use the "feedback" link on the bottom of Fossfor.us to send a note to our team.