Web 2.0 blogs
I am always going on here about things you need to be reading and sites/blogs you should be visiting if you are interested in the Web from a (new) business angle. Here are a few of the top ones again:
Techcrunch: founded on June 11, 2005, Techcrunch is a weblog dedicated to obsessively profiling and reviewing new Internet products and companies. If you want to know what is new out there, what is getting funded by venture capital, and about cool stuff that works, this is a must read site.
37signals company blog: is about entrepreneurship, design, experience, simplicity, constraints, pop culture, 37signals products (Basecamp, Campfire, etc.), and more. If you don't know these guys and their products you don't know jack. They also have a separate blog about their products.
Read/Write Web: provides Web Technology news, reviews and analysis. It began publishing on April 20, 2003 and is now one of the most widely read and respected Web 2.0 blogs.
Update:
I am adding John Battelle's Searchblog which to me is the source on search (he wrote the book on Google - literally) and a lot more.