Chris Whealy's RT60 spreadsheet has a tab of Absorption Coefficients
SAE's Absorption coefficients of common building materials and finishes
WSDG Absorption Coefficient Chart
8.3 Table of Absorption Values
Acoustic Benefits of Plants in Buildings
Physikalisch-Technische Bundesanstalt 2000 data sets excel file, includes several walls
Leather chairs
Zip of 450 Absorbtion Coefficients by mhsoft
SAE RT60 calculator - view HTML for absorbtion coefficients
STC transmission resonance vs coincidence gifs.htm
a-Databank Working Group 1.73 Room Acoustics
jonessy's RoomAcoustics program's absorption data
Sabin 3.0 - Absorption Material Databases

The below three are the property of StudioTips

Sabin Data text file
Sabin Data sorted
Short list of absorbtion coefficients

No longer available

Absorption Coefficients of Common Building Materials
Absorption Coefficients of Common Building Materials