The Tulip framework is open source and available from SourceForge. See Tulip's homepage.
A plug-in is a piece of code that deals with a specific problem such as:
node or edge metrics (values assigned to node and/or edges);
graph layout (node and/or positions);
graph clustering, through the use of subgraphs (Tulip can handle a whole hierarchy of subgraphs).
node/edge betweeness centrality (used in social networks analysis) (Freeman, Brandes);
(node) clustering index (Watts and Strogatz)
Tree layout algorithm (Reingold and Tilford or Walker's algorithm)
force-directed layout algorithm (GEM by Fricke et al. for instance)
k-core decomposition of (scael-free) graphs
bisected k-means clustering algorithm adapted to graphs
Access Tulip API documentation or look at the source code to become familiar with the API and learn how to implement such algorithms. Once a plug-in is completed you just add it to the proper directory to make it available through the default GUI.