The 2001 Mars Odyssey launch was transmitted live on NASA TV. Several organizations inside and outside NASA transmit NASA TV to the World Wide Web. Note that a free plugin is often required to view the video stream, and it is highly recommended that you install and test out the plugin well in advance of the launch. Both video and audio coverage is provided.
In addition, online news organizations, such as CNN, MSNBC, Fox Online and SPACE.com, may carry some launch and mission coverage. This is just a partial list of non-NASA sites that may carry the NASA Television signal on the Web from time to time.