Software for system architecture capture
My first engineering leader used HUGE pieces of paper for initial system thinking. He insisted all forms of interaction be shown on that one big diagram

If nothing else, go to a grocery store and get/buy a 5' piece of butcher paper. 

Don't be afraid to use the other side for Version 2, once the Rev zero diagram gets too messy and evolved and altered.

Humans are powerful visual thinkers. Get the butcher paper.