Some tools and whatnot I find handy:

Twitter (obviously) - definitely one of those things that a lot of people can't see the point of, but once you can figure out a way in which it can work for you, it can be invaluable. I've extended my online networks immeasurably through it, and often with people I'd never otherwise have got into contact with. Tips: Regularly prune your follower/following lists // Explore reputation- and recommendation-based networking through things like Follow Friday // Search on terms and #hashtags that interest you/you want to find out about // Listmine other people's follower/following lists for interesting-sounding tweeters.

Dropbox - useful (free) online storage with desktop synchronisation.
https://www.dropbox.com/

Aardvark - a 'social search engine' - basically a Jeeves that works! When you sign up, you say what things you are interested in/know about. Then when someone has a question they want answering they ask Aardvark; Aardvark then sends the question via email/IM etc to people its magic alogarthims identifies as most likely to know the answer. I've had a 100% hit rate on questions I've asked - and the human/fuzzy logic element means even if you get detail wrong in your question, you can still get a correct answer back.
http://vark.com/

Podomatic - free podcast hosting/blogging site. Run by podcasting enthusiasts, easy to use, a bit shonky at times, but an excellent way in to the world of audio blogging.
http://www.podomatic.com/

SoundCloud - audio-sharing site, very good for spoken word and music mixes
http://soundcloud.com/

Scribd - document-sharing & archive; very handy for publishing document scans, and for finding them
http://www.scribd.com/

 out of  found this valuable. Do you find this valuable?