One of the few things that I'm really missing in C++ is a good reflection facility. Reflection is a valuable tool in many situations - for example as a support for serialization or marshaling in distributed systems, efficient object-relational mapping, debugging/logging, generic programming and meta-programming, etc.
This is the main reason why I started to work on a portable open-source C++ reflection library called Mirror. This library is developed with the intention to be proposed for inclusion into the Boost C++ library.
Currently Mirror is somewhere between alpha and early beta version. The sources and the partial documentation can be downloaded from the Boost Vault or checked out via svn from the Boost Sandbox. It's http interface also allows browsing the sources and examples and see the docs online.
There is also a C++0x version of Mirror at sourceforge and the latest online documentation is available at kifri.fri.uniza.sk.