Magento – What Default Modules I can disable

This really depends.

In cases you just want to hide your module output it is enough to go to Magento admin System \ Advanced \ Advanced and disable your module output there.

In case you want to disable your module completely you will need a file access but even then you may get some surprises. Some modules are almost impossible to disable completely :-)

First and easiest way is to set <active>false</active> in your module bootstrap file.

However this will not eliminate your classes if some other modules are extending them. Deleting module files is also not an option as you will still need to remove extension declarations to avoid fatal errors. Also as Petar mention in comments if your extension is implementing custom backen model or something like this and there's also some entities relying on this model deleting extension files will again result in fatal error(s).

Additionally deleting files is not removing any database entries extension created during execution of install/update scripts and/or extension's functionality.

1. Yes it does. First of all, less modules means less code to (potentially) load and process. Next to that, a lot of modules, like for example the Mage_Rss module run a lot of code in the background like forcing reindexes on certain events.

   On the method best to use: disabling a module using System > Configuration > Advanced only suppresses the output of a module while still including the code of that Module in the shop. This is handy when you don't want a modules functionality but you need it's Models or Blocks for example because other (3th party) extensions depend on it. Disabling it using app/etc/modules/*.xml will completely remove it from the installation so performance wise this is the best option.

2. I usually disable the following extension via XMl

   • Mage_Rss
   • Mage_PayPalUk
   • Mage_Tag (when not used in a project)
   • Mage_Poll (cause who uses polls anyway)
   • Phoenix_Moneybookers
   • Mage_Sendfriend
   • Mage_Rating (when not used in a project)
   • Mage_Bundle (again, if not required by client)
   • Mage_Downloadable (see above)

   and via System > Congiguration > Advanced the Mage_Adminnotification which suppresses those annoying popups in the backend.

   You can probably disable several more core extensions depending on what you're using or not. Just make sure that you don't compromise the stability of Magento. I guess this will take some trial and error.