Is there an organization or association which a professional developer should get involved with? There are local .NET user groups, for example, but I'm thinking more about nationally/globally recognized groups which might help one's career or even personal development. Or perhaps even groups which simply provide industry discounts/perks on such things as online technical books subscriptions the way ACM does. Any suggestions?
Try the BCS.
BCS is the leading professional body for those working in IT. We have over 65,000 members in more than 100 countries and are the qualifying body for Chartered IT Professionals (CITP). We are also an international awarding body for a wide range of qualifications for both users of IT and IT specialists.
.NET user groups are practically hobby groups in comparison. These professional bodies are concerned with business development, IT working to enabling businesses and similar - ie focussed on what you can do with technologies, rather than the technologies themselves. If you want to become a true professional developer you need to realise there is much more to IT than just writing code (in any system).
I visited LinkIn's Group Directory today and seached on "asp.net" 30 results were returned. Not sure if any of the groups are worth joining though.