With the question re-phrased as two parts:

"I know that there an open source operating system and a web server, what else is there?

Almost anything you've heard of there being software for. Probably any piece of commercial software you have and like, or have seen and want.

Where can I find it?"

The quickest and most reliable way is to use Google. Try queries like "open source X" where X is the thing you already know and like.

If you add a third question:

"I think there ought to be a program that does Y, but I've never seen it before. Where do I find something like that?"

Then the problem is a little harder. Google may help (as above), but it's also possible you've got an original idea. If you hang out with the right kind of programmers and buy them beer, good things might happen to you.

Come to think of it, that's a good way to get your first two questions answered without resorting to Google, as well. :)

Freshmeat.net has a respectable list of open source projects, and obviously there is SourceForge. SF has major spam issues on their forums, and both have some painfully outdated and stale projects. Unfortunately there is no F/OSS App Store.

The grand-daddy of free software registries, still going strong, is Freshmeat. They formulated and refined the “trove classifiers” system of multi-dimensional categories used on many other such registries.

Huh. I have to admit that I don't know of one. With several hundred thousand projects available - libraries and so on included - I'm not even sure where one would start with that.

You might give Ohloh a look: it's an automated project indexer, that provides intelligence around project activity and so on.

But even Ohloh - or governance vendors like Black Duck, Open Logic, Palamida, etc - won't have truly comprehensive lists of all open source software. There's simply too much of it, spread across too many repositories and sites.

For the Mac - http://www.macports.org/ - package installer with 6328 packages available at this instant in time. Command line driven, requires XCode, compiles from source, but generally works well. Have to be a little bit of a geek to make it work but nothing too strenuous (if I can do it, anyone can).

Debian's among the most comprehensive, but will clearly exclude a.) very new, unpackaged software (it's missing Hadoop, for example), and b.) packages and software not designed for the Linux platform. Still a great resource though, and for those wishing to wade through it, the Debian Popularity Contest should help.

I would almost always answer this question with the question "open source software that does what?" Or, is the question intended to prove to their management team that "everyone is doing it". I hope we're getting to the point were the latter is less important.

By far not complete, but that's the trick: Some of the most famous packages around and listed by themes:

http://osliving.com/index.php