I am a very inexperienced Arduino user, and I was trying to interface an Arduino Duemilanove to an ETT DS1307 I2C Real Time Clock. I took a long time to work out that the library downloaded from http://code.google.com/p/sjunnesson/downloads/list should be saved to Arduino-0018\libraries and NOT Arduino-0018\hardware\library.
This solved the error when I tried to run the example in the library I downloaded from the website above:
92: error: DS1307.h: No such file or directory In function 'void setup()':
In function 'void loop()':
I know this may not be much help, but I hope someone benefits from it!