SELECTED ANSWER
replied on April 22, 2014
While Workflow doesn't have a "Current Logged In User" token (since multiple users can log in at once, how would WF know which user was the right one?), Workflow does have the ability to collect a user name token when a user performs an action.
For example, if you use a Wait for Entry Change activity to pause your workflow until a specific action occurs the activity will produce a token for the user who satisfies the wait condition:

As Devin mentioned, you also have the Initiator token available to give you the username of the user that started the Workflow.