Essentials

• Brixcc – on Windows, nothing beats the combination of Brixcc and NQC when it comes to compatibility of all retro MindStorms pbricks! Recommended for anyone.
• NQC – Not Quite C. A compiler for LEGO bytecode programs that can be used for the RCX, NXT, Spybotics and Scout. Uploads firmware, downloads datalogs, etc. A must have!

Community

• Eurobricks -an active community with forums about everything LEGO.
• Philo‘s MindStorms – a huge amount of information and inspiring creations.
• Using NQC on a Raspberry Pi to Program a Lego Mindstorms RCX Brick
  
• Cybermaster Touch Sensor Multiplexer on the NXT  (works on the RCX too!)

Internals (advanced)

• Lego Mindstorms Internals – Russell Nelson’s collection of RCX internals and more links!
• RCX Internals – Kekoa Proudfoot’s awesome page about the internals of the RCX.
• RCX Manual – very detailed information on the workings of the RCX by Ole Caprani
• Programming the RCX

DIY (homebrew sensors etc.)

• An Infra-Red Link Using an AVR
• Homebew lego sensors
• Adding hardware features/improvements to the LEGO Mindstorms RCX