Several years ago I was using it in production at a major CDN. I would assume they still use it.

According to Wikipedia, at least the Eve Online MMO uses it quite heavily, as well as an IronPython Mail client thingy. For whatever that's worth.

As a programmer on the EVE team for the last 7 years, I would not recommend it at this point in time. Stackless is a fork of CPython, maintained by a small number of people. I'd rather look at pypy which has stackless support: http://pypy.org/features.html#stackless and is under active development.

Or look at golang which has the same type of primitives(microthreads, channel based communication and scheduling) built in.

I've searched extensively for years for something python-based which enables actual multi-core concurrency.. and after spending one week with golang, I strongly prefer go compared to anything I've found with python- multiprocess, stackless, pypy, etc. None of them let you reap the benefits of hardware like coroutines with communication channels in golang.