Source Models List
When you are doing custom magento extension with admin configuration sometimes you need to make custom drop down options. To create new custom dropdown menu you have to make source model, however magento by default have a lof of them. By clicking Read more you will find the most completed list of useful soruce models. It will help you save time and use existing source model with options instead of making new one.
Usually source models defined in system.xml file of new extension.  Lets review for example system.xml file of the Reviews default magento module:
 AS you can see on the line 43 frontend type secified as select, it means that admin option will be as dropdown option. On the line 44 you can see that source model which contains options for this field specified as adminhtml/system_config_source_yesno . As we can see at the list below it shows Yes/No options. This way this field will be dropdown with Yes/No options.
Here is list of the most used source models with options:
Model Description
adminhtml/system_config_source_admin_page returns list of all the items in Magento ® Admin menu
adminhtml/system_config_source_allregion returns list of all country regions grouped by country
adminhtml/system_config_source_catalog_timeFormat returns time format (12h or 24h )
adminhtml/system_config_source_cms_page returns list of CMS Pages
adminhtml/system_config_source_cms_wysiwyg_enabled returns list of text editor options
adminhtml/system_config_source_country returns list of all countries from directory module
adminhtml/system_config_source_cron_frequency returns list of cron frequence ( Daily, Weekly, Monthly)
adminhtml/system_config_source_currency returns list of currencies
adminhtml/system_config_source_customer_group returns list of customer groups
adminhtml/system_config_source_date_short returns list of available short date formats
adminhtml/system_config_source_design_robots returns list of robots options (index, follow)
adminhtml/system_config_source_email_identity returns list of email sending identities (General Contact, Sales Representative, etc)
adminhtml/system_config_source_email_method returns list of email methods (Bcc, Separate Email)
adminhtml/system_config_source_email_template returns list of transactional email templates
adminhtml/system_config_source_enabledisable returns list of two options (“Enable” and “Disable”)
adminhtml/system_config_source_frequency returns list of frequencies (Always, Hourly, Daily, Weekly, Monthly, Yearly, Never)
adminhtml/system_config_source_locale returns list of locales
adminhtml/system_config_source_locale_country returns list of localized country names
adminhtml/system_config_source_locale_currency_all returns list of localized currency names
adminhtml/system_config_source_locale_timezone returns list of localized timezone names
adminhtml/system_config_source_locale_weekdays returns list of localized weekday names
adminhtml/system_config_source_notification_frequency returns array of hours frequency ( used for cron)
adminhtml/system_config_source_notoptreq returns list of three options for customer widget mandatory indetificator (“Not”, “Optional”, “Required”)
adminhtml/system_config_source_order_status returns list of available order statuses except pending payment
adminhtml/system_config_source_payment_allmethods returns list of all payment methods
adminhtml/system_config_source_payment_allspecificcountries returns boolean values for payment ("All Allowed Countries", "Specific Countries")
adminhtml/system_config_source_price_scope returns list of price scopes
adminhtml/system_config_source_shipping_allmethods returns list of all shipping methods
adminhtml/system_config_source_shipping_allspecificcountries returns boolean values for shipping ("All Allowed Countries", "Specific Countries")
adminhtml/system_config_source_shipping_taxclass returns list of tax shipping classes
adminhtml/system_config_source_store returns list of stores
adminhtml/system_config_source_tax_basedon returns list of values which tax based on
adminhtml/system_config_source_yesno return boolean options (“Yes”, “No”)
checkout/config_source_cart_summary return list of display options at the cart
 Let's also review other settings for the field.
  • label - Its label of the field we making (Line 41)
  • sort_order -  Position of the field. In case there are few fields, then this field will be used to order of fields. (Line 44)
  • show_in_default - Show field for the default store scope (Line 45)
  • show_in_website - Show field for website scope (Line 46)
  • show_in_store - Show field for store view scope (Line 47)
This options also means same for lines 33-38 But in this case it will applied for settings group.
Reviews admin configuration options
You can also check out field we discussied at the System->Configuration->Catalog tab -> Product reviews Tab
This will help you to make custom admin configuration field with dopdown option values for  which already exist in default magento installation.