There are many books and tutorials that share useful code snippets for WordPress. For example, you can find hundreds of custom functions right here at DigWP.com. You can also find them in my WordPress books, tutorials, and code snippets. For many code snippets and custom functions, the usage instructions will say something like:

As part of our ongoing mission to always find new ways to improve our operation, we’ve set up a new system for downloading DigWP book updates.

The new system is much more user-friendly and is designed to simplify things and make the upgrade process easier for everyone.