The file js.gp contains the formula of Igusa's invariants. 
You can compute them for an arbitrary polynomial using gp.

First start gp in the directory containing the file js.gp, 
and then enter the polynomial. For example:

? p=x^6+3*x^4+5*x-42 

or 

? p=a*x^5+4*x-b

Then type

? \r js.gp