The NIST/TRC Web Thermo Tables (WTT) Professional Edition is an online database of critically evaluated thermochemical properties. There are two versions of the database, the Professional (with all the data) and the Lite (with 150 selected commonly used compounds). We subscribe to the Professional version.
Presently, the Professional version of WTT contains data for 7729 compounds. Data includes critical properties, vapor pressures and boiling temperatures, phase transition properties, volumetric properties, heat capacities and derived properties, transport properties, reaction state-change properties, as well as index of refraction, surface tension, and speed of sound. Various search options and interactive data plotting capabilities are provided via the web interface.
This is a subscription database (not the same as the freely available NIST Chemistry WebBook). The two products are complementary however, and each one has internal links to the other.