Databases

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

Tutorials

Molecular orbitals for polyatomic systems
http://www.chemistry.mcmaster.ca/esam/Chapter_8/section_6.html
Molecular orbitals for polyatomic systems.

Huckel Theory
http://www.chem.qmul.ac.uk/software/download/mo/6.pdf
A short tutorial on Huckel theory.

Software

SHMO - Simple Huckel MO Theory Calculator
http://www.chem.ucalgary.ca/SHMO/
Freeware that performs Hückel molecular orbital calculations with a simple graphical user interface.

WebMO
http://www.webmo.net
Freeware that provides a graphical user interface for popular electronic structure software (such as GAUSSIAN, GAMESS, and MOPAC). The computational engine must run on a UNIX or Linux server, but the WebMO interface allows for users to generate input and obtain output from any networked computer.