That's a good question- and the answer is "from all over the place" including:

• Word of mouth from friends
• Some blogs, podcasts and feeds I subscribe to
• http://www.makeyougohmm.com/
• http://thenextweb.org/
• http://www.makeuseof.com/
  
• Journals and magazines
• Directories
• Search Engines

Here are some of the better Web 2.0 directories (in order of importance in my opinion):

Go 2 Web 2.0
This is by far the best directory out there, though it may be a little slow loading.




Ziipa
Another good one, though not quite as comprehensive.






Cnet Webware
Some of the news mentioned here isn't relevant to Web 2.0, but it's a great place to keep up with the latest trends!





Startup Meme
New startups tend to have some creative applications out there for us to use.





Mashable
Mash-ups are combinations of two or more web 2.0 tools designed to provide interaction for specific uses. Some of these are incredible!