For those occasions where you want to make a custom alteration of a Drupal form, do some lightweight theme switching, or make a function available on every page, a very small custom module may do the trick. The examples listed below are too small and simple to be worth contributing as full modules, but can provide useful functionality or be the basis for your own development.
Modules listed here will not generally add or change any database tables, so you can just upload them and enable them though the admin >> modules page on your site.
Important: as with all PHP you use on your Drupal site, errors in module code can cause your site to go offline, damage your database, or cause server problems. These are for use by more experienced site administrators. Do not test these on a production site, and review at least the basics of how a module works in the Module developer's guide before using any of this code.