Ability for users to annotate their racks in some way.

I use Modular Grid to explore and research future modules. I use the racks to group modules under exploration and use the rows as default categories by shuffling modules around the rows as I learn more about them. This alows me to see all modules under research all in one view rather than in seperate racks.
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-- John Jacobs
+1
Exactly what John described.
MG is the #1 module research tool for me.