10.1 official changes list (2,889)
A comprehensive developer-oriented list of bug fixes, additions, and changes in OS X 10.1. Mostly of interest to developers, but there's a ton of detail on the areas that were affected by the 10.1 update. An impressively detailed document!
Apple on web database engines (2,936)
A very detailed page describing mySQL and postgreSQL, including instructions for securely installing both database packages.
Apple's Cocoa developer docs (2,035)
With the release of the December Dev Tools, Apple updated over 700 items on this page that previously containd no information. If you want to know more about Cocoa development, this looks like a great resource.
Apple's Darwin Open Source Page (2,065)
Apple's new consolidated entry point for all things Darwin. Hosted as part of the Apple Developer website, this URL now contains everything you need to know about working with Darwin.
Apple's KBase Recent Changes (2,518)
A KBase (KnowledgeBase) page listing all recently changed articles (finally!). Apple has also improved other areas of the KnowledgeBase, including a personal page for tracking articles of interest.
Apple's KnowledgeBase (3,764)
Apple's support pages for current products, known as KnowledgeBase, have been tremendously improved recently. You can now set preferences, there's a recent changes page (linked separately in this list), you can save favorite searches or articles, etc. If you haven't looked at it lately, it's worth a visit.