NIST Chemistry WebBookFrom the site: The NIST Chemistry WebBook provides users with easy access to chemical and physical property data for chemical species through the internet.
The data provided in the site are from collections maintained by the NIST Standard Reference Data Program and outside contributors. Data in the WebBook system are organized by chemical species. The WebBook system allows users to search for chemical species by various means. Once the desired species has been identified, the system will display data for the species.
CRC Handbook of Chemistry and Physics, 98th Edition by 
CRC Handbook of Basic Tables for Chemical Analysis by 