Here are a few of the sites I have found helpful for professional purposes:

evcom crystal

EVCOM is the body for our industry in the UK

Crystal Interactive is a leading provider of audience response systems
istock itn
iStock photo is a great source of royalty-free images ITN Source is a great place to find footage
economist linkedin
The Economist is a great source for research into global and market economic trends

How To Really Use Linked In is a useful resource for all of us who joined Linked-In and are now not quite sure how to use it properly!

zen active

I recommend the book of this name by Garr Reynolds for anyone who wants inspiration and practical tips on presentation design.

Chris Davidson is a good presentation trainer.