Do I really need to elaborate on that? Wanna learn something? Just Google it!
Twitter is my favorite learning tool. I call it my professional-development-on-the-fly. People or instances I follow contribute to my daily knowledge acquirement. Links to posts, websites or other materials are tunneled 24-hour a day to my Twitter page. I suggest you not limiting your Twitter usage to entertainment only. I agree its great to follow movie stars and singers but at the end of the day they do not contribute to your development at all.
You really get videos of everything on YouTube. The number one video sharing portal has 13 hours of videos uploaded every minute! Apart from music videos and movie trailers, you are sure to get a great share of training materials as well. Great deal for visual learners, like me.
On Slideshare.net you can browse and/or upload PowerPoint presentations. Most of the topics I looked for up to now were found on this portal. Going through slides is a quicker way to acquire knowledge compared to reading thousand of words in a book. You can either download the slides, share them or embed on your site/blog. On Slideshare you can also get ideas on creating attractive PowerPoint slides for your presentations.
5. LinkedIn Groups
I've joined several groups of interest on LinkedIn where I interact with professionals around the world. Topics or questions raised by anyone in the group are answered and debated by numerous experts in the field. Here you can learn from professional expertize in a collaborative way. Discussions in groups are presented in forum-like structure, which makes them easy to follow and participate.
What is your favorite online learning tool?