Registry operator of .co domains, .CO Internet S.A.S., announced that ".co domains are now available through registers such as GoDaddy.com, Register.com, Network Solutions, and others."
For all practical purposes, one of the biggest advantages of the .co TLDs, besides the fact that they're similar to .com TLDs, is the fact that they're already widely recognized. More than 20 countries, including the UK (.co.uk) and Japan (.co.jp), already use them for their country-specific domains.
GoDaddy is selling .com domains for just $29.99/yr. Twitter, acquired t.co doman, and is currently using it as a link service.