Google:
Official Google blog
John Battelle's Searchblog
Google Blogoscoped
Unofficial Google blog
InsideGoogle blog
MSN Search's blog
Yahoo! Search blog

Machine Learning in the Valley
Venture Evolved (blog by engineer turned VC in Chicago)

Microsoft:
Mini-Microsoft blog
Technical Careers at Microsoft blog
Raymond Chen's blog

Programming: Jeff Atwood's Coding Horror

Security: Krebs on Security

Alexis Madrigal (The Atlantic)