Arduino

Permanent installation with veroboard

Excellent tutorial. Again, I had no idea.
Another walkthrough

Weather station

WX station receiver, plus links to others; interesting 433 Mhz helical antenna designs
Another WX station

DHT22

Calibrating a DHT22 sensor

433 MHz

Antenna examples

Pi problems

It's almost like I wrote this... Python script running on pi, unable to handle paging request.

Solar power

DIY Solar cells

I had no idea.

Serial comms

Serial over NRF24L01, which links to:
Serial comm fundamentals with Arduino. Choice quote:

A note on strings

The Arduino provides two methods of storing character data.  You have
the standard C style character arrays, and you have Arduino's own
attempt at a C++ String class.  C style character arrays require a bit
more effort on the programmers part while Arduino's String class
attempts to provide an easier to use object that handles most of the
string manipulation under the hood.  There is one potentially
significant pitfall with the String class that you need to be aware of
when using it.  The class relies on dynamically allocating buffers to
handle different string sizes as well as changes in string sizes.
With the Arduino's limited SRAM, repeated dynamic memory allocation
will fragment memory and eventually cause the Arduino to behave
unpredictably or lock up completely.  It is for this reason that I
utilize C style character arrays.  I'm not saying the String class
can't be used, but steps need to be taken to keep it from fragmenting
memory at which point you lose much of the advantage the class tries
to provide.