I'm very impressed by what Joel, Hartmut and the rest of the folks have done with the 2.1 release of Spirit. The syntax has been improved, the code is much neater and more powerful – bravo guys!
If you use C++ and have a need for a parser do yourself a favour and check out Spirit.
Spirit 2.1 will be in Boost 1.41.0, which is due for release any time now (it's currently in beta). Until then you can read the pre-release documentation.