EMCC have some interesting things on their web site, especially if you are considering Symbian development.
The Technical articles page includes introductory articles on Mobile JSRs, Symbian Signed, IMS, Wi-Fi, VoIP, UIQ v.3, Exchange ActiveSync, Push-To-Talk, Push Email, S60 v3 and Symbian OS v9.
The Developer Zone has full example source code from the book Developing Series 60 Applications (beware it’s pre-Symbian 9.1/S60 3rd Edition so it’s a bit old now), a download for highlighting Symbian keywords in Visual C++ 5/6, a download to allow viewing of Symbian types in the Visual C++ debugger, and CLINDEX, a command line tool to examine Symbian libraries and their exports.
By the way, if you are considering Symbian development it’s best to start with Carbide.c++ Express rather than battle with SDKs under Visual Studio. I have used Carbide.c++ heavily on three client projects now and as of version 1.2 it became stable and useful enough to replace Carbide.vs and CodeWarrior for general S60 and UIQ application development.