Well, what do you need it to do? I have been working on a pure Python instrument library that can communicate over several different protocols (serial, VXI-11 (LXI), GPIB (linux-gpib), and PyVISA) called python-ivi, located here: http://www.github.com/alexforencich/python-ivi . It is an interpretation of the Interchangeable Virtual Instruments specification. It's not VISA, though, so if you specifically need VISA, it's not going to help you.
It's written in python 3 and tested in linux, but it should be cross-platform as it is pure python. The implementation of VXI-11 in python-vxi11 is also pure python and therefore should also be cross-platform.
The project is still in relatively early development in terms of the higher-level drivers and abstractions, but it does basically everything PyVISA does for low-level interfacing.