Stefano Garuti

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/

Registration Widget

This plugin provides you with a new Widget that inserts the Registration Form in your sidebar (or wherever a widget-area is defined in your theme).

dytri

This small plugin helps you to do on your site can register and authorize users without the standard form of WordPress, and as to issue a block to display the logged user’s site in the form of “Hi, User!”. The plugin include short codes, that you can use on your pages. You just prescribe the correct code and the page will automatically display the specified form. Initially, D-login is not using their styles, so you have to set up their own so-, as you will need. D-login the perfect choice for your website, all the more so, that it is completely free!
<h4>ShortCodes</h4>
<ul>
<li>dloginuser – displays the login form</li>
<li>dreguser – displays a registration form</li>
</ul>
<h4>Features</h4>
<ul>
<li>Authorization</li>
<li>Registration</li>
<li>Password recovery</li>
<li>Antispam</li>
</ul>
More information can be found on the <a href="http://dytri.com/shop/dlogplugin" rel="nofollow ugc">home page plugin</a>.

D-login

This small plugin helps you to do on your site can register and authorize users without the standard form of WordPress.

Registration Widget vs D-login