I've checked out both concurrentlua and luaproc in the past and they seem quite similar (both use cooperative scheduling via co-routines). I initially liked luaproc more because the channel-based message passing approach in luaproc seems to me to be inherently more flexible than message passing based on process id's. luaproc leads to a particularly elegant implementation for the chameneos-redux benchmark, for example.
concurrentlua seems more maintained, though; it showed up on luaforge more than a year ago and its latest update was in May, which to me means that the creator is interested in continuing the project. I believe this is the paper that introduced concurrentlua:
Thanks! I was looking for the ConcurrentLua PDF - it isn't linked on the project site anymore.
IIRC, luaproc was put up on luaforge because people on the mailing list read the paper and started asking if the source was available.
There are also about a half dozen other concurrency packages for Lua, which at first seems rather odd. As a language, it has quite a few semi-official extensions, but unlike Scheme (with its standard and several implementations), it's because the language designers are very serious about keeping the core language small and portable for embedding. You can import the ~800k of source into your project tree and fork it as necessary, compile it down to a ~200k library, etc. In practice, this means that people contribute Lua extensions that help it can work nicely with whatever funky C++ game engine they're using, etc., as well as more general libraries.
concurrentlua seems more maintained, though; it showed up on luaforge more than a year ago and its latest update was in May, which to me means that the creator is interested in continuing the project. I believe this is the paper that introduced concurrentlua:
http://docs.google.com/gview?a=v&q=http://www.google.com...
luaproc's luaforge page is here, for comparison's sake:
http://luaforge.net/projects/luaproc