What is the pH of acid rain?

Rainwater with a pH value lower than 5.6 (the pH value of unpolluted rainwater) is known as acid rain.  Rainwater with a pH as low as 2.5 has been recorded in some parts of the world.

4-4.5 See the reference for lots of details.

Normal, clean rain has a pH value of between 5.0 and 5.5, which is slightly acidic. However, when rain combines with sulfur dioxide or nitrogen oxides—produced from power plants and automobiles—the rain becomes much more acidic. Typical acid rain has a pH value of 4.0. 

It can vary depending on your location, but pretty much anything with a pH less than 5.4 or so down to about 4 has been measured and is considered to be acid rain.

generally not seen as "acidic" until the pH falls into the 4 range or lower.  Rain with pH in the low to mid 5 range is "normal", unavoidable.  Strictly speaking, from a scientific definition, any rain which has a low pH because of presence of acid-forming contaminants is "acid" rain.  It gets a bit complicated though in detail because CO2 is the main driver of that slight acidity in nature, and CO2 is partially a contaminant (not all is of natural origin), so where would we draw the line?  Also, there is natural variation in some "contaminants" such as sulfur oxide gases and even local CO2 concentrations, and thus while there is an ideal pH for rain, natural rain is often not ideal in behavior.

Thus, we usually only speak of rain as "acid rain" when it is significantly acidic, which is anything less than about pH=5 (getting close to an order of magnitude more acidic).  Rain can get down into the pH=2 range if there are nearby high-level discharge sources for the strong acid-forming gases of NOX and SO2/SO3 (like a smelter stack with poor pollution control systems).