The government's Consumer Price Index (CPI) is the main index used to track inflation. Many claim it under reports inflation in many ways. It would be nice if there were other indexes that could test the CPI. A new one is MIT's The Billion Prices Project
(BPP) which builds an index based on ~5 million items sold by ~300 online retailers.
This My Money Blog post
has a good review of this index with two charts that compare the CPI with the BPP. It's interesting to note that in the last several months, the BPP has been higher than the CPI.