These are great additions for workflow activities from the codeplex site.
Send Email with HTTP File attachment - Allows sending emails with attachments retrieved using a web request
Send Email with List Item attachments - Allows sending list item attachments as files attached to an email
Start Another Workflow - Starts another workflow associated with a list item
Grant Permission on Item - Allows granting of specified permission level on a spicified item
Delete List Item Permission Assigment - Allows deleting of specified permission level assigment for a given user
Reset List Permissions Inheritance - removes any unique permissions assigned to an item by inheriting list permissions
Is User a member of a SharePoint group - Checks if a given user is part of given sharepoint group
Is Role assigned to User - Checks if a user role is already assigned on the current list item
Lookup user info - allows to lookup properties in site's user information list for a given login
NEW! Copy List Item Extended Activity - Allows copying/moving list items and files cross site.
NEW! Send Email Extended - Enhaced version of the OOTB activity. Allows you to specify the sender. Also does not break links in body.
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Workflow migrate code. Quite nice for dev to production activity.