Craft store scores again...
Here's yet another example of a great little tool you can pick up at a local craft store. (see our previous write-up on other craft store scores here).
This cutting tool is great for cutting straight lines in foam and also allows you to maintain enough control to do some free-wheeling cutting for going along curved edges and such. The cutter is made by Fiskars and is fairly inexpensive.
The I found myself able to cut along both a straight edge as well as just following lines on the foam. I made quick work of a few foam sheets to form even width strips for things like Chernobyl style ants and other patterns that need rectangular pieces of foam.