I looked at some options some time ago. Here are some:
- Boost Thread - This is the most standard choice. Boost is the most standard library for C++, that is not in the official standard.
- POCO - Has thread support and a lot more. Is my preferred choice because it lets you set thread priorities, something boost doesn't support. Thread priorities are important for my application domain (soft real-time).
- Zthread - Looks a good library. I have no experience with it.
- ACE - Well known library. I have no experience with it.
Then you have libraries that let you operate at an higher abstraction level like Thread Buildings Blocks.