The Banteay Srey Butterfly Centre (I think it used to be called the Angkor Butterfly Center) is on the road to Banteay Srei and a worthwhile stop if you like butterflies (as I do).
It is a small space but packed with lots of butterflies and cocoons and even a large "stick insect" (at least when I was there).
For most people I would imagine it is a quick 15-30 minute stop, but quite nice. The staff person that showed me around was knowledgable and provided interesting information (I often don't find staff at these types of places useful). This one claims to have a social purpose also, of providing jobs to local people and based on the knowledge of my guide I found that credible - the staff seemed to care about the site (which is often not the case).
Butterfly cocoons (all photos by John Hunter - see lots more photos at Curious Cat Travel Photos)
The butterfly center is also near the Cambodia Land Mine Museum.
Location: Pradak Village (25 km from Siem Reap) on the road to Banteay Srei temple.
Website for the butterfly center
Related: see 1 minute Curious Cat Bantaey Srei video