Allows editing request headers. This will affect most of the plugins in a task (everything that uses urllib2).
Check out cookies plugin as well.
To use feeds that require login, setting cookie is usually required. You will need to extract the cookie for site and grab details from it (often uid, pass).
headers: Cookie: "uid=3945; pass=f8bae52ca325b14ef2523fabc"
Setting headers in a task will affect most of the HTTP request made in the task. This means that the values you set in the headers can be leaked to some unintended services as well.