The Weather Channel from TV - a wealth of useful information about when it's going to rain (and you can get your samples), as well as information on the past month's rainfall. Not the best for historical rainfall data, but very good for current month, past month, and current / future weather conditions.
Excellent site that contains a lot of past weather data, including precipitation amounts and other storm event data. Can't recall how much it rained on the date you took a stormwater sample, or when it began or ended raining? This is the place for you!