Ruby and Upgrading to Leopard
I recently upgraded my mac to OS X 10.5 and had some problems installing the Ruby gem Hpricot. This particular gem is compiled from source when it is installed (on a Mac at least, no idea about Windows or Linux). When I tried to install it, it kept throwing an error and failing. The problem was that I had forgotten to also upgrade Xcode, so it was being compiled with an old copy of gcc.
FYI - Leopard comes with Hpricot preinstalled on Leopard