From this page you can download an Excel® spreadsheet that allows you to do Hückel calculations for conjugated hydrocarbons. So a long as you have Excel® on your computer that will run the Visual Basic macros, you should be able to run the calculations on fairly good size molecules.
Among the features are:
(Hopefully) Clear instructions on how to use the spreadsheet.
Sample H-Matrices for Various Compounds
The Energy Levels, Eigenvalue Matrix, and Eigenvector Matrix (albeit with round-off errors)
Total pi-Energy (β units), Heats of Formation and Heats of Atomization (in eV, kJ/mole, or kcal/mole)
Resonance Energies from Two Parameter Polyene Reference Structures with Provisions for Calculating Schaad and Hess Resonance Energies
Resonance Energies Per Pi-Electron (β)
And of course there are the usual disclaimers that 1) the program is accepted "as is" (that is, no guarantee it will work on your computer), 2) the users accept all responsibility for anything they may do with the program, and 3) the author of the program cannot necessarily (and regrettably) respond individually to questions about implementing or maintaining the program.
Plus all the other stuff that's in computer disclaimers.
But at least the program is free.
The program is based on a two subroutine macro that must be enabled for the program to work. The first part handles input and output and the second subroutine is the Jacobi transformation. The latter subroutine should be adaptable for general diagonalization of square symmetric matrices as it is self-contained and not requiring any other routine for it to work.
So if you want to try your hand at Hückel calculations, you can download the spreadsheet by clicking here.