Since the MSDN documentation for Default Custom Action Loacations and IDs is unreliable (they look to have copy+pasted the information from the SharePoint 2007 documentation), our only sure bet is to browse the file system and find the custom action IDs and groups ourselves.

I had need of this information today, and found the items from the site settings page located at:

C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\FEATURES\SiteSettings\SiteSettings.xml

For those looking specifically for it, here is the custom action for “Delete this site”:

<CustomAction
    Id="DeleteWeb"
    GroupId="SiteTasks"
    Location="Microsoft.SharePoint.SiteSettings"
    Rights="ManageWeb,BrowseUserInfo"
    Sequence="40"
    Title="$Resources:core,SiteSettings_DeleteWeb_Title;"
    Description="$Resources:core,SiteSettings_DeleteThisSite_Tooltip;">
    <UrlAction Url="\_layouts/deleteweb.aspx" />
</CustomAction>

If you’re looking for that custom action, you’re probably wanting to hide it. Here’s the xml you’ll need in your elements file:

<HideCustomAction
    Id="HideDeleteWeb"
    GroupId="SiteTasks"
    HideActionId="DeleteWeb"
    Location="Microsoft.SharePoint.SiteSettings">
</HideCustomAction>