I believe there's no real risk-free rate.
Risk-free rate refers to an interest rate without default risk, which does not exist in real world.
People use the concept of risk-free rate to quantity the price of default risk on the of the base rate (i.e., the credit spread represents the additional risk of default in corporate bond).
Edited:
The explanation here is easy to understand.
http://www.investopedia.com/terms/l/libor-curve.asp