Custom action.

Links about how to create custom actions:
Sample CustomActions for SharePoint
EditControlBlock (ECB) menu item in SharePoint Wiki and Blogs
Creating Hierarchical Menus with a CustomAction in SharePoint
Custom Actions - Simple Steps to Add Your Touch to Site Actions
Custom action schema from Microsoft:
ContentTypeId = "Text"
ControlAssembly = "Text"
ControlClass = "Text"
ControlSrc = "Text"
Description = "Text"
GroupId = "Text"
Id = "Text"
ImageUrl = "Text"
Location = "Text"
RegistrationId = "Text"
RegistrationType = "Text"
RequireSiteAdministrator = "TRUE" | "FALSE"
Rights = "Text"
Sequence = "Integer"
ShowInLists = "TRUE" | "FALSE"
ShowInReadOnlyContentTypes = "TRUE" | "FALSE"
ShowInSealedContentTypes = "TRUE" | "FALSE"
Title = "Text">

Attribute	Description
ContentTypeId	Optional Text. Specifies the ID of a content type to associate with the custom action.
ControlAssembly	Optional Text. Specifies the assembly of a control that supports the custom action.
ControlClass	Optional Text. Specifies a control class that supports the custom action.
ControlSrc	Optional Text.
Description	Optional Text. Specifies a longer description for the action that is exposed as a tooltip or sub-description for the action.
GroupId	Optional Text. Identifies an action group that contains the action, for example, "SiteManagement". If contained within a custom action group, the value of the GroupId attribute must equal the group ID of the CustomActionGroup element.
Id	Optional Text. Specifies a unique identifier for the custom action. The ID may be a GUID, or it may be a unique term, for example, "HtmlViewer".
ImageUrl	Optional Text. Specifies a virtual server relative link to an image that presents an icon for the item.
Location	Optional Text. Specifies the location of this custom action, for example, "Microsoft.SharePoint.SiteSettings". If the custom action is a menu item or toolbar button, then the possible options include EditControlBlock, NewFormToolbar, DisplayFormToolbar, and EditFormToolbar. If contained within a custom action group, the value of the Location attribute must equal the location of the CustomActionGroup element.
RegistrationId	Optional Text. Specifies the identifier of the list or item content type that this action is associated with, or the file type or programmatic identifier (ProgID).
RegistrationType	Optional Text. Specifies the registration attachment for a per-item action. Possible values include: ContentType FileType List ProgId
RequireSiteAdministrator	Optional Boolean. TRUE to specify that the item be displayed only if the user is a site administrator; otherwise, FALSE. Using the RequireSiteAdministrator attribute for the drop-down menu of Windows SharePoint Services commands associated with list items is not supported.
Rights	Optional Text. Specifies a set of rights that the user must have in order for the link to be visible, for example, "ViewListItems,ManageAlerts". If not specified, then the action always appears in the list of actions. To specify multiple rights, separate the values by using commas. The set of rights are grouped logically according to AND logic, which means that a user must have all the specified rights to see an action. For a list of possible values, see Microsoft.SharePoint.SPBasePermissions.
Sequence	Optional Integer. Specifies the ordering priority for actions.
ShowInReadOnlyContentTypes	Optional Boolean. TRUE if the custom action is only displayed for read-only content types on the page for managing content types. The default value is FALSE.
ShowInSealedContentTypes	Optional Boolean. TRUE if the custom action is only displayed for sealed content types on the page for managing content types. The default value is FALSE.
Title	Required Text. Specifies the end user description for this action.

The most useful thing in Custom action url from: Parameters(Microsoft)

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To build a custom action that points to the edit form of a single list/doc lib when selecting a list item you need to pass the itedmid and listid in the urlaction url.

Use the registrationId if you only want to have the feature for lists based on that particular content type.

<urlaction url="">url?ID={ItemId}&List={ListId}"/>
</urlaction></customaction>

To build a custom action that points to the edit form of a single list/doc lib when selecting a list item you need to pass the itedmid and listid in the urlaction url.

Use the registrationId if you only want to have the feature for lists based on that particular content type.

<CustomAction
Id="MyId"
RegistrationType="ContentType"
RegistrationId="0x0101[Guid]"
Location="EditControlBlock"
ImageUrl="/_layouts/images/EDIT.GIF"
Sequence="106"
Title="Edit Procedure">
<UrlAction Url="~site/MyList/Forms/EditForm.aspx?ID={ItemId}&amp;List={ListId}"/>
</CustomAction>

<urlaction url="">javascript:window.location= '{SiteUrl}/_layouts/mypage.aspx?List={ListId}&Source=' + window.location"/></urlaction>