NLib has a lot of sophisticated NURBS math tools. It was literally written by one of the men who wrote The NURBS Book.
As far as I know, it doesn't do anything much for trimmed NURBS surfaces, in the sense of a surface trimmed by an arbitrary series of curves. (Might be a tessellation routine for that case?) Basically it is mostly geometry, with only very limited topology support.
There are lots of products which provide geometry & topology support: SMLib, SOLIDS++, Acis, and Parasolid come to mind. The first two are built on NLib. I don't think any of them have as sophisticated NURBS geometry tools as NLib does. (Except to the extent that you can call NLib routines directly with the first two.)
I'm not aware of any commercial product which directly competes with NLib at all. Which doesn't mean one doesn't exist! I'd be interested if anyone has experience with other libraries.