On the PC:
Digsby - can be made semi-portable via http://www.mcsoft.online.fr/madcow/?Ninja-Digsby.html
    Facebook
    GMail
    Google Chat
    Twitter
    Linked-in
    NOT Skype

Trillian - I didn’t see a portable
    Facebook
    GMail
    Twitter
    Skype
    Google Chat
    NOT Linked-In

Meebo - doesn’t do Skype, LinkedIn, or Twitter

On the iPhone:
    Beejive - does Google Talk and Facebook
    ?