http://daz3d.com/
Apparently for the month of Feburary 2012, Daz 4 Pro, Bryce 7 Pro, and Hexagon 2.5 are all free for download. These are the full versions, not some sample thing.
These programs are definitely not push-overs and you can probably compare them to 3ds Max and Maya.
The only catch is that you have to create an account on their website(and get email news letter for free), but beyond that, nothing else. Upon adding them to your cart there, a full price discount will be applied at checkout and then they will email you the serial codes.
For anyone who wants to mess with this stuff, this is probably the greatest opportunity you will have for a professional program beyond actually buying one.(or the other more shady option...)
So far I have played with Hexagon a little. While it doesnt export .nif files, it will export .3ds files and .obj files, both of which Blender can import. Hexagon doesnt seem as powerful as Blender, although I have only played with it a little so I am not terribly familiar with what it can do compared to Blender.
I have yet to take a look at the other programs, but from what I have read, Daz and Bryce are definitely good programs. If there isnt a way to export .nif files from either of them, then its almost certain that they will export in a format that Blender can import and convert to .nif.
As of this post, there are about 12 days left to get these for free. I read on another forum that these are free for the month of Feburary, but I didnt see exactly how long from the site itself, so I will assume this is probably correct. At worst I am wrong and you in fact have more time to get them.