Right now I just set up a spreadsheet on google docs. I made the cells all squares, I put a C in a system where I have a colony, O if I have an outpost (or multiple outposts) with no colony. P in a square where there's another player where I don't have a planet and I'm not trading with them, T in a system where I'm trading with a colony in the system, A for anomaly, eventually there'll be E if they're embargoing me so I can't trade, W for war, etc.

Eventually I might modify it to add more info, player name, etc.