My experience in this area is relatively limited (being relatively new to libraries!) but I am a member of two organisations - CILIP and the Cambridge Library Group. My reasons for joining them both probably stemmed from the fact I believed it would 'look good' to be able say I have interests in my career outside of my day to day work. With CILIP this really remains the case, as I have not used my membership wisely. I guess to me it seems quite terrifying and big! The CILIP website was of great help to me in gaining the position I have today (which they accredit) and I do enjoy flicking through their monthly 'CILIP Update' magazine. As a trainee I paid so little for my membership I guess I haven't felt the need to really get involved, which is definitely not a good thing. Maybe this thing should prod me to investigate a bit more.
The Cambridge Library Group has been of more use to me whilst I have been a trainee, giving me the opportunity to expand on the number of libraries I have visiting. In the past 6 months I have been to the Archive and Conservation Laboratory at the Sedgwick Museum's Brighton Building, the Radzinowicz Library of Criminology, the Fitzwilliam Museum libraries and the Needham Research Institute all thanks to my membership of this group. I have also spoken to some interesting people working in business and public libraries, and it has given me a chance to meet up with a few of the other trainees on a more regular basis.
I had a little explore of some of the informal organisations listed, beginning with The Library Society of the World. It confused me a little though as all the posts on the front page were rather dated! I had a further explore of the LIS New Professional Network, which as I have already mentioned in Thing 6 I found quite interesting to read what people were discussing, and thought about making my own comments. I also think the downloadable resources are a great idea, and enjoyed looking at 'how to use twitter' and reviews of various LIS master courses (even if it is too late to change my mind now!).
What I would love to find would be an organisation for scientific librarians - reading and getting to know other professionals who are doing something I would be keen to go into would be useful. Maybe it would give me some more ideas of where my career could go.