If Red Hat Enterprise Linux 5 is too expensive for your blood, CentOS 5 is now available. CentOS 5 is immediately available for i386 and x86_64 though PowerPC, IA64, and SPARC support is planned for the future. The release announcement is available here. One of the changes for CentOS 5.0 is being completely rebuilt using a newer build system. The release notes are available from the CentOS Wiki.