The GNOME project always welcomes new contributors. Whether you are a seasoned developer or you have never written a line of code in your life, you can probably find some way to contribute to and improve GNOME. For help finding the best way for you to contribute to the GNOME project and for help getting started, read Joining the GNOME Project.
The current status of GNOME is available here. There is a development schedule as well as TODO lists and contact points for various components of GNOME. If you are looking for a task that needs doing, this is a good place to start. GNOME work is often coordinated on the GNOME and GTK mailing lists, and newcomers are encouraged to join one or more of the lists.
Also, please check the GNOME Software map to see or add programs already available for the GNOME desktop. If you find some program of interest to you, you might want to contact the author(s) of the code and offer your help directly.
If you want to help out with the GNOME API Reference Documentation, visit the Helping Out With GNOME API Documentation page for instructions and the steps to take.
If you want to hack GNOME, consider checking out the SVN tree.
If you have system administration experience, and feel that you can help out with the dirty-work of keeping the gnome.org services running smoothly and efficiently for the benefit of the whole GNOME community, please visit the GNOME System Administration site.
If your company or institution can offer free software job openings for the GNOME desktop and other GNU software projects, please submit job listings to jobs@gnu.org. You can read some technical arguments for supporting such software in The Cathedral and the Bazaar. This paper was behind Netscape's decision to release its client software source code. The Free Software Foundation maintains the Free Software Job Page for listing opportunities for paid employment.