As you know WordPress’s Registration Form is only displayed in it’s own page, and if you want to invite a user to register, the only option you have is to provide a link to the registration page.
With this plugin you have a new widget that insert a Registration form in your sidebar.
The new registration form supports all the hooks that the standard Registration page does, so you can use it safely even with other plugins like Registration Plus that add fields to the form.
UPDATE and NEWS at: http://www.wpmania.it/registration-widget/