http://threadpool.sourceforge.net/
I've tried it some time ago and wasn't too happy with it. I have to admit I didn't try really hard, but I usually had cores unused. I'm not sure if this was a problem of the threadpool or of my code, but anyway, from browsing their CVS repository on SF the project looks pretty dead, since the last change was two years ago.
Hmm, thanks for the report. Must admit I've seen not-great performance from Boost.Threads synchronization too.
Just to steer this back toward topics Boost, note there is also Oliver Kowalke's Task library in the works. Since it uses Context it should be lighter weight.
Sounds like TBB is the way to go for now though.
Cheers
Gordon