The Gephi wiki provides detailed information about a wide variety of topics, and includes user manuals, plugin information, community details, and much more.
The Gephi blog provides periodic updates on major news about Gephi.
Other Network Visualization Tools
Cytoscape is an open source network graph software that was originally designed for biological use, but now has an expanded scope. It is available at http://www.cytoscape.org/.
Jung is a java-based open source network graph framework at http://jung.sourceforge.net/.
GUESS is a Python-based software for graph exploration at http://graphexploration.cond.org/.
Pajek is a Windows-based program designed for large network analysis at http://pajek.imfm.si/doku.php?id=pajek.
GraphML is one of the older graph projects available, with fewer recent updates than most of the tools in this list at http://graphml.graphdrawing.org/.
D3 is a JavaScript-based tool that contains some network-based components http://d3js.org/.
SigmaJS is an alternative to D3 offering some similar options for creating network-based visualizations at http://sigmajs.org/.
Ora is another tool designed for network analysis and graphs, and is part of the CASOS project at Carnegie-Mellon. You can download Ora at http://www.casos.cs.cmu.edu/projects/ora/.