This page contains links to sites I mentioned in Xcode Tools Sensei and helpful Mac OS X development sites. I was originally going to put this material in an appendix for Xcode Tools Sensei. Because this material consists of links, I figured that placing the links here would be easier for you than typing URLs from the book.
Apple's developer site contains lots of documentation and sample code to help you write Mac OS X and iOS programs.
Apple has many mailing lists where developers can post questions about Mac development and receive answers to their questions from Apple's engineers and other readers. There's an Xcode mailing list for questions about the Xcode Tools. Most of the questions are about Xcode, but you can ask questions about other tools. There's a performance and optimization mailing list for questions about Instruments. There's also an OpenGL mailing list for questions about the OpenGL Tools.
CocoaDev is a wiki with lots of Cocoa programming information.
Apple's Instruments application provides a front end for DTrace. The DTrace.org site has lots of information on DTrace, which will help you if you want to write custom instruments for Instruments. The Dynamic Tracing Guide is especially good.
The git home page is the first place you should look for information on the git version control system. You can download git from the git home page as well as read documentation and find git GUI clients.
Intel's developer page has documentation and articles about their processors.
The LLDB site has the most comprehensive information on the LLDB debugger. If you use the LLDB console, you should visit the LLDB site to get the most out of LLDB.
MAS (Mac SVN Server) is an open-source tool that simplifies installing a Subversion server on your Mac. Check out MAS if you want to set up a Subversion server for your office.
The OpenGL official page is the place to go to read OpenGL specifications. It also contains message boards where you can post your OpenGL questions.
PyObjC lets you write Cocoa applications in Python.
The Subversion home page is the first place you should look for information about the Subversion version control system. Among the things you'll find on this site are Subversion installers, Subversion GUI clients, and an online Subversion book.