Have any of you ever used them (or emulated them with Assembly)? Do you know of any program making use of them?

It's the first time a hear of them. Never used, never saw.
Seems like a Windows "exotic" api.

http://en.wikipedia.org/wiki/Fiber_(computer_science)

Fibers are an extra layer of abstraction/overhead on top of multi-threading.
http://msdn.microsoft.com/en-us/library/ms686919%28v=vs.85%29.aspx

Since they are not optimal, there's no reason to use them in with ASM (assuming you use ASM for speed/size optimization).

I've never seen them used, but I'd guess there's some cross-platform threading library that works POSIX *nix and WIN32, which makes use of the fibers api.

I use them at work as part of the hardware simulator I help to write/maintain. The entire system models hardware as (very many) virtual threads using a cross-platform library that uses fibers and whatever the unix equivalent is. This has some useful properties, as only one thread is actually executing so the need for thread safety is reduced and it allows for easier debugging because everything is serialised. (also tracking the state of 100's of threads simultaneously is not practical )

I read about fibers first time in this book ( great one btw , recommend to read to all win developers )

http://www.amazon.com/Programming-Applications-Microsoft-Windows/dp/1572319968

Chapter 12 is about them ( not to much text ),
with small example program in C++,
though didnt used myself , so cant help you ...

http://msdn.microsoft.com/en-us/library/ms682661.aspx

The Old New Thing has nice article(s) about fibers.

Thanks for your answers. I've just found something I didn't know about: User-Mode Scheduling. The minimum requirements are a little bit aggressive though... UMS is probably more heavier than fibers but at least you'll have far less problems with code assuming they run on actual separate threads and if you get blocked the system will call you back so you can pick a new UMS thread (with fibers you were forced to avoid this situation and perhaps it is not always feasible to do)

Iirc MS SQL Server can run in some fiber mode, but other than that? Haven't bumped into software using fibers (that I know of, anyway).