If you are using an IDE to develop your applications (who doesn't these days?), you have probably stumbled on a lot of files that are automatically generated which you don't really need to add to source control. If you are using Visual Studio and plugins/extensions (such as ReSharper and NuGet) these are quite some files. The ".gitignore" file was designed to solve just that: putting it in the root of your source control directory will prevent "git add ." commands from adding those files/directories. My current file is based on this one from github, with extensions from this one from stackoverflow, and finally a line that I added for ReSharper user files.
You can download the file here (be sure to remove the ".txt" extension).
Note: this is basically a repost from my previous blog.