A9.com building in Downtown Palo Alto. From their About page:
A9.com, Inc. researches and builds innovative technologies to improve search experience for e-commerce applications. A separately branded and operated subsidiary of Amazon.com, Inc., A9.com opened its Palo Alto, California, doors in October 2003. A9.com’s technology powers search on Amazon.com and other web sites.
Oh hey, and like most Silicon Valley companies, they are hiring!