Koders.com is a search engine that searches a large library of open source code. It's also a product you can purchase and point at your own codebase, but it's the easy access to all manner of reusable code, in just about any language, that has my beanie propellor whirling.
You can rather quickly find out how other developers have used method X in a given API, for instance. Seeing .NET examples in use in the wild is a much more potent educational tool than than the skeletal examples on MSDN, personally speaking.
You can also look for working examples of a concept. How have others approached solving problem Y? A browse through the relevant search results can show you real code being used in real life. Because honestly, if you are faced with a tough nut to crack, chances are someone else has already cracked it.
It gets better, because not only can you learn from your research, you can pick it up and use it wholesale. Or at the least, adapt the solution to your specific needs.
So the ability to search for code is, alone and all by itself, very useful. But you can you also browse through entire software projects! Holy crap. I spent some time just grazing:
- DotNetNuke, an open source web application portal framework.
- Subsonic Project, a toolset that helps a website build itself
Koders also provides a Firefox search engine plug-in, and a Visual Studio plug-in: Koders Plug-Ins. I am cozying up to both.