I published a plugin for Gephi, an open source graph data structure visualization and analysis platform. The plugin calculates a minimum spanning tree of a graph data structure via Kruskal’s Algorithm.
The plugin can be installed like any other plugin on the Gephi Marketplace. To be explicit, install and open Gephi, click the 'Tools' menu, click 'Plugins', click 'Check for Updates', click 'Available Plugins', and search for 'SpanningTree'. Check the checkbox, click 'Install'.
I implemented the bulk of the project years ago in UW Madison's Introduction to Algorithms Computer Science course. In my local experiments the algorithm returns with correct results within in a minute even when the graph sizes approach Gephi's default memory limit. That said, the plugin has issues. Rather than hide the warts away, I opt for Clay Shirky's publish-then-filter paradigm so that others can lend perspective to which problems are most valuable to address. I look forward to feedback from the community on areas for improvement.
 Chapter 10 (Failure for Free). Shirky, Clay. Here comes everybody : the power of organizing without organizations. New York: Penguin Press, 2008. Print.