Oooo, what's MapStat? Is that a 'cheat'?
MapStat, which is part of the CivReplay Suite (CrpSuite) is a utility program that tracks selected in-game information. It is a free download and can be found in the CivIII Mods subforum.
It, and CivAssistII, default to spoiler free and I have not used either one any other way.
I use the trading and happiness screens most often.
Trading shows the known civ, how many turns are left on the most recent trade deal, how many workers the civ has to trade, their gold, what luxuries/resources that civ has to trade, which ones I have to trade, which technologies they have that I can buy and which ones I have that I can sell them. I only know if something can be bought or sold. It doesn't tell me if they will sell it or for how much.
The other common utility, CivAssistII, has more trading details than MapStat does. It will show the nuts and bolts of each ongoing trade deal, even if you have several deals with one civ that were made on different turns. MapStat does not do this. It just indicates how many turns are left on the most recent trade.
I could go to each civ every turn and find out this information (every turn) but MapStat does that for me.
The Happiness screen is useful since it shows which cities are unhappy right now and gives me a chance to fix that before the end of the turn, if possible. Cities in resistance are also noted.
MapStat uses less resources than CivAssistII, which makes it more attactive on older machines (Win98, 500 MHz, 128 RAM or so) though CivAssistII is much prettier.