Lightweight, portable C++ fibers, MIT license
I actually blogged about this in the past. Have a look! I hope it answers your questions. In it, I cover a number of libraries, and I was particularly interested in ones that were useful for systems programming (asynchronous IO).
Conspicuously absent from that coverage is Boost.Coroutine, which I'll discuss here. Boost.Coroutine may be considered "heavyweight" conceptually (in terms of its family of types), but the implementation is quite efficient. The real problem is that Boost.Coroutine is incomplete, and (last I checked) far from complete. I had spent some time trying to work with the author through its non-starter issues, as I was really looking forward to using it in conjunction with Boost.Asio (this was one of Boost.Coroutine's primary objectives), but the author has not had the time to take his work to the Boost formal review stage.
list of implementations for C
for ultra lightweight "threads" take a look at Protothreads at the bottom of the wikipedia article.
- Libtask: MIT License
- Libconcurrency: LGPL (a little tighter than MIT, but it's a functional library!)
Both are written for C.
If Boost seems to heavy, helpful people have extracted the relevant parts of Boost (fcontext) as a standalone library, e.g. deboost.context.