The Nexus Mod Manager (NMM) is a desktop client that facilitates the downloading, installation and management of video game modifications that the Nexus sites support.
This project page will provide you with the source code for NMM.
NMM is released on the copy-left GPL open source license and we're always looking for help in developing the client. Whether you want to help in major ways or provide minor tweaks is up to you. If you're interested in helping out drop us a note on our discussion board. I'm (Dark0ne) also available for chats on MSN, AIM and Astra.
For further information about helping out please read this thread