Wednesday and Thursday are special because the Expo Hall is open. There is where you find a hundred or so companies and non-profit Exhibitors who have booths or tables. Here is where you get to see and meet the presidents and top designers/programmers in the industry.
Languages: One highly popular web programming language is PHP and there are hourly PHP Sessions on this. You can listen to the inventor of the PHP language describe what is new and how to best use its features. This language is what most CMSs such as WordPress are written with.
Data: There are hourly Data sessions on various databases such as the popular MySQL which is the back end for WordPress and most other CMSs. Also present was the popular Postgres / PostgreSQL database, also open source.
Hardware: The best place to see open source hardware is at the Maker Shed (booth) where Arduino projects are displayed and sold. Actual developers of open source hardware will be there showing their projects in a Mini Maker Faire.
Secret book discount for O’Reilly books: Order direct from OReilly.com using the code TSSCN12 and receive 40% off print books, 50% off ebooks. Free shipping within U.S. for all orders over $29.95. (Not sure how long this will be valid)