what library is able to extract SIFT features in Python?
OpenCV is free to use. 
But SIFT itself as algorithm is patented, so if you would make your own implementation of SIFT, not based on Lowe`s code, you still could not use it in commercial application. So, unless you have got a license for SIFT, no library with it, is free.
But you can consult with patent guys - some countries like Russia does not allow to patent algorithms - so you can you SIFT inside such country.
In python which library is able to extract SIFT visual descriptors? I know opencv has an implementation but it is not free to use and skimage does not include SIFT particularly.