Ubuntu Customization Kit is a tool you can use to customize the official Ubuntu Live CDs by adding additional language packs, applications and so on. It doesn't just work for Ubuntu (GNOME), but also for Kubuntu / Xubuntu and Edubuntu.
A new Ubuntu Customization Kit version (2.2.1) was released today. Here's the changelog:
- GUI now uses Kdialog, Zenity, Dialog or builtin (in that order) if available.
- Brasero style checksums support was added
- Bugs with virtual packages for languages packs were solved
- Workaround for portuguese (pt_PT) language management was implemented
Download Ubuntu Customization Kit 2.2.1 (it obviously comes in an Ubuntu .deb package but source files are also available).
For a web-based tool to customize the Ubuntu Live CD, see Reconstructor. Most users will probably find Reconstructor easier to use, but there are some limitations (read the Reconstructor post).