Calculators:
Convertion of Temperature and General Units from Inst. of Chem., FU Berlin
Astro-Physical Calculator by Chris Dolan, U Wisconsin
Convertion of General Units by Frank Tapson, University of Exeter

Chemical databases:
Chemical Charge by Janet L. Kaczmarek, SUNY Stony Brook (Create an electrically neutral ionic compound by varying the cation to anion ratio)
Molecular Modeling for Chenical Education, by Lebanon Valley College
Sheffield ChemDex
ChemWeb

Crystals and crystal structures:
General Crystal structures: fcc (ccp), hcp, bcc, diamond, etc by Lebanon Valley College
Crystal lattice structures by Naval Research Lab
TriPhase by Melanie Rebosa, SUNY Stony Brook (Crystalization of minerals from a melt)
Bilbao Crystallographic Server, Materials Laboratory at UPV/EHU
     Table of Space Group Symbols
     Space Groups: Generators and General Positions (GENPOS)
     Space Groups: Maximal Subgroups (MAXSUB)
     Space Groups: k-vector types (KVEC)
     Space Groups: Supergroups of a Given Type (SUPERGROUPS)
     Space Groups: Irreducible Representation (REPRES)
     Space Groups: Wyckoff Positions (WYCKPOS)
     Wyckoff Positions Splitting (WYCKSPLIT)
     Point Groups Tables
     Group-Subgroup Lattice and Chains of Maximal Subgroups
     Pseudosymmetry Search in a Structure (PSEUDO)
     Correlations Between Space Group Representations (COREL)
     IR and Raman Modes (SAM)
     Inelastic Neutron Scattering Selection Rules (NEUTRON)

Electron structure and electron interaction with atoms and solids:
Diffraction: Bragg's Law by Konstantin Lukin, SUNY Stony Brook (How waves reveal the atomic structure of crystals)
Electronic Structures Database by Naval Research Lab

Fundamental Physical Constants and Conversions:
NIST Reference on Constants, Units, and Uncertainty
By Tom Knight, MIT Artificial Itelligence Lab
From Institute of Chemistry, FU Berlin
For programmers: C header file by T.M. Sanders
For programmers: FORTRAN data file by T.M. Sanders
Physical Constants and Astronomical Data by Chris Dolan, U Wisconsin
SI Units from Institute of Chemistry, FU Berlin
Dictionary of units by Frank Tapson, University of Exeter

Molecules:
Tight-Binding Molecular Dynamics by Naval Research Lab (Tight-Binding Parameters and Scalable Molecular Dynamics Tight-Binding Codes)
Some well-known molecules
Korea Thermophysical Properties Data Bank (properties of pure materials, vapor presures, etc)

Periodic Table of Elements:
Tight-Binding parameters for the Elements by Naval Research Lab (equations of
state, elastic constants, vacancy formation energies, phonon frequencies, and surface energies for the elements)
by University of British Columbia
WebElements^TM