I love CmapTools.
This program allows you to freely link concepts together in a simple-to-use, free program with a lot of powerful features like synchronous editing (for maps hosted online,) suggestions (that offer linked-concepts from other maps) and an outline view (with a list of the concepts made, prepositions used and linking phrases.)
How is this useful for a med student? Here's a few maps that I've made last night using this program -- the little icons illustrate the ability to link resources to the map!
It is very useful for visualizing mechanisms and learning where your knowledge is deficient!
Here's a wikipedia link to a list of concept map programs that you can check out.