First go to: http://get.live.com/mail/overview, then click
'Sign up', once your one the page put this code in the your browsers address bar e.g. mozilla, ie, opera:
Ok, where it says 'idomain' that is the drop down option which means it is 'hotmail.co.uk', so where it says 'put the new name here' you put 'thesite.com', it doesn't have to be 'thesite.com' it could be anything e.g. porn.net, youtube.com etc.
Ok so once you've entered that into the address bar and clicked enter it should say 'Successful', now the option should be different and should show 'whatever you put.com';
If you want to do it to another site just view the sites source (right click>view source), and find something similiar to:
<select id="idomain" name="idomain" class="DomainList">