Google indexes not just web pages and documents, but also interesting file types like executables, databases or source code files. To search for a file with a specific extension, you need to use the operator filetype.
1. Find applications.
If you search for [winamp filetype:exe], the first result is a direct link to Winamp 5.08. Searching for [python filetype:msi] brings many direct links to Python 2.4 setup. It's interesting to see that Google actually indexes the files and gets some metadata from them.
2. Find source code.
There's a lot of code available on Google: some on governmental sites, other useful for educational purposes. You can even restrict your search to a certain type of license.
3. Find databases.
You can find CSV files, SQL scripts, DBF files, MDB files.
4. Find feeds.
Get a list of great feeds to import to your favorite feed reader. This way you can search for the feed address of your favorite site, without looking in the source code.
5. Other interesting files.
Registry files, configuration files and, my favorite, Inno Setup scripts.
So the next type you need a file to test an application or to use it as an example, try Google.