it was a utility from a hackers site that use to do that, but you can open it with other editors as well. The editor from Bualders Gate I think use to open that file as well, but it has been a long time since I played with that stuff.
I nolonger have a link to the site I got it from as it was 3-4 years ago (and now 2 PCs) since I played with that.