Databases

CRC Handbook of Chemistry and Physics
http://www.hbcpnetbase.com/
Data may be accessed after purchasing a personal or institutional subscription.

NIST Chemistry Webbook
http://webbook.nist.gov/chemistry
Contains databases of thermochemical properties of a large number of substances.

F*A*C*T Database
http://www.crct.polymtl.ca/FACT/
Useful collection of phase diagrams.

Thermophysical Properties of Fluid Systems
http://webbook.nist.gov/chemistry/fluid/
Thermophysical Properties of Fluid Systems.

Tutorials

Mixtures; Partial Molar Quantities; ideal solutions
http://www.chem.arizona.edu/~salzmanr/480a/480ants/mixpmqis/mixpmqis.html
The equations behind ideal solutions, mixtures and partial molar quantities.

Phase Diagrams
http://www.science.uwaterloo.ca/~cchieh/cact/c123/phasesdgm.html
Gentle introduction to phase diagrams.

Phase Equilibria Menu
http://www.chemguide.co.uk/physical/phaseeqiamenu.html#top
Comprehensive coverage of phase equilibria including Raoult's Law.

Phase Rule
http://pruffle.mit.edu/3.00/Lecture_26_web/Lecture_26_web.html
More information on the phase rule.

Software

Henry's Law Constants
http://www.henrys-law.org
Extensive compilation of Henry's Law constants, including an online calculator for conversion between units.