We also use the software from Safe Engineering Services. It's pretty expensive. (Each module costs around $5000 to $6000 as I recall, of which you'll need at least two.) However, if you are going to be designing ground grids regularly, I wouldn't recommend anything else. I might add, the tech support at SES is OUTSTANDING.
I know the people that make ETAP software also have a ground grid module. I'm sure it's less expensive. However, I've never used it.