Read Arduino pin from another Arduino [duplicate]

This should be relatively simple IMO.

If you want to use analog, then the code for Arduino 1 (Writer) would be as follows:

byte myByte=0;
void setup(){
pinMode(3,OUTPUT);
}

void loop(){
if(somethingHappens){
myByte++;
}
analogWrite(3,myByte); 
}

And Aruino 2 (reader) would read it as follows:

byte readByte;
void setup(){
Serial.begin(9600);
}

void loop(){
readByte=analogRead(0);
Serial.println(readByte);
}

Normally, this should work exactly like you would think, and nothing will be damaged. However, some people choose to put a resistor between the pins, just in case a programming error should accidentally set both pins to be outputs. It's not neccesary, but if you do anything where one wire is used bidirectionally and devices change input/output roles it is a good idea. Otherwise, you should be fine.

Also, Stuyvenstein's answer might give slightly odd results, because AFAIK the analogWrite() function on almost all arduino board variations is implemented with hardware PWM, not true DAC. You can use an RC filter to get real analog from the PWM though. IIRC the frequency is something like 490Hz.