A cahce tool would allow a user to temporarily store a snapshot of inline data from previous run of the module.
Imagine a browse tool that was inline as opposed to a terminus tool (input and output). Now allow that browse tool to persist its data after a run of the module. When an option on that tool was activated, it would block all of the dependent tools upstream from it and instead send its cached data downstream.
The reason I think this would be a useful tool is that I often come to the end of creating a module when I'm working on the Reporting tools. I run multiple times to see the changes I've made. When the module has a lot of incoming data and complex data transformations, it can take a long time just to get to the point where the data gets to the reporting tools. This cache tool would eliminate that wait.