I've been using Charlie Lee's excellent Google Spreadsheet Bitcoin tracker sheet for awhile but it pulls data from a single exchange at a time and relies on the ordering of those exchanges on the bitcoinwatch.com site, which vary with volume.
I figured out I could get better numbers more reliably from bitcoinaverage.com, which (predictably) averages multiple exchanges over various time periods. They offer a great JSON API, but unfortunately Google spreadsheets only export JSON -- they don't have a function for importing it.