You can register ANY name that fits the following criteria:
Less than 63 characters long
Is not an existing ICANN TLD (so that using Handshake doesn't break your internet browsing experience)
Is not an Alexa top 100k domain name (only google.com can register .google)
Contains alphanumeric characters (A-Z, 0-9) and/or dashes. Unicode characters are supported via the punycode format.
Search for any name you want on the Namebase Domains page.