Arduino Mega 0 - 10V analog input?
Input voltages can be "scaled" down to suit the Arduino's ADC input voltage range.
 As Oli says, a 10V input can be scaled down with a potential divider.
 In its most basic form this can be two resistors.

As shown below, the input current flows through R1 and through R2.
 Voltage drop across a resistor = V = I x R
 As both resistors carry identical current (= input current)
 the voltage drop across each resistor is proportional to the  resistor value 



So, if R1 = R2 then the the voltage across each is equal so they each drop half the input voltage. 

In the general case - input voltage is across R1+R2. 
 Output voltage is across R2 only.
 So Vout = Vin x R2 / (R1 + R2)

ie the division ratio = R2 / (R1 + R2) 

When R1 = R2 the division ratio is 1/2 so
 a 0-10V input will be reduced to o-5 volts. 

R1 and R2 can be typically 10k ohms each in this case. 

Much larger values can be used but they can affect the conversion accuracy. eg 100k + 100k.
 Usually it is wise to limit R2 to not more than about 20k ohm.