This is a good source for current rates, and current and past yield curves. (Be sure to check out The Living Yield Curve.) Review the current factors influencing bond prices and yields under Bond Market Updates. (
http://www.smartmoney.com/bonds
)
Access historic bond yields from the FRED database (of the Federal Reserve Bank of St. Louis). (
http://www.research.stlouisfed.org/fred/data/irates.html
)