Skype - what can I say, free phone, view of who is 'live' on-live, conference calls and a messaging system all at the click of a button!
Trello - great medium for developing ideas, managing projects and developing marketing plans!
Twitter (inc Tweetdeck) - thought this was going to be useless - but is very dynamic, feels alive when compared with the very slow response you get from the other networking applications.
Good but must try harder:
LinkedIn - going in the wrong direction - removing links to other media - seems to be reverting to recruitment site mode.
Storify - I like this one personally but not been a great interest for others.
Scoop.it - you need to make sure it doesn't end up as a dumping ground - the stuff you put in the loft thinking it would be of use someday!