More Winter 09 documentation has been released. Scott Hemmeter wrote a post listing some of the pages containing new content, but I wanted to go a bit in-depth on those and some other parts of developer.force.com.
Documentation
This should be your first stop whenever you have any questions about ANYTHING on the Platform. It has sections on Web Services API (formerly just called API, to distinguish it from Metadata API), Metadata API, Apex, Visualforce, AJAX, Office Toolkit, Force.com Migration Tool, IDE, and the Library.
Core Resources
In addition to a super Documentation page, force.com has a new section which contains, well, resources sectioned by the Platform’s service categories: Logic (Apex), User Interface (Visualforce), Database (Objects, formulas, triggers, etc.), Integration (API, REST), Services –What? We now have Services as a Service?–(Workflow), Packaging and Distribution (AppExchange), Development (Metadata), and Tools (IDE, Force.com Builder, Data Loader).