I am wondering if there is a modifier or tool that will essentially undo a subsurf or subdivide command.
You can use the 'Decimate' modifier set to 'Un-Subdivide' mode:
Increasing the number of 'Iterations' will increase the number of times the modifier un-subdivides the object.
This works particularly well with quad based geometry. Below, the monkey on the left (that had a Subsurf modifier applied) has 8,000 vertices, while the un-subdivided monkey on the right (with 2 iterations) has 2000 vertices, with all details preserved well.
What I found today is that you can also use Ctrl+E and there's an 'Un-Subdivide' option but I don't know if that's the same as the modifier.