Here's a strange one:
If you annex a city containing a Guild you haven't built yet, you acquire that guild and *all of its accrued points*. For example, I got an Artists Guild from a city capture, and 5 turns later got a GA from it. This can be really awesome for conquest. Saves you building the guild and shaves many turns off your first GA/GW/GM. But, it can have unintended consequences, like preventing you from building the guild in the city you wanted to build it in. Unfortunately, I don't recall seeing the Artists Guild in the puppet screen. I don't think it appeared until I annexed. :-(
I suspect this is a bug, as the Guilds are essentially National Wonders and shouldn't transfer over.
I think someone mentioned some of these, but here are some tricks to figure out which civs are in the game:
1) Ask all players to DoW someone. It'll show the people you haven't met as greyed out options.
2) Click DoW on all CS. Don't accept. It'll show unmet civs who are protecting them.
3) If you see borders revealed by a map ruin, or just inside fog-of-war, and don't have the color memorized, or can't see it clearly, you can sometimes hover over that tile to see who owns it.
Other tricks:
* Hovering over tiles in areas of the map that you can't currently "see" but have revealed in the past will in some cases update them with information you shouldn't have. ;-)
* You can tell if someone has Fertilizer, Economics, or Chemistry by their tile output yields. This will tell you what they're researching... and how far along they are towards artillery, most importantly.
* You can tell who's roughly ahead in tech even if they're not #1 by which era they're in.
* You can tell who went a different tech path from you by hovering over a potential trade route with them. If you get 4 beakers from someone, and they get 3 beakers from you, you know they went a different route. It's not as obvious when you're just flat out behind. This can be useful when deciding who to steal from.
* Sometimes it's good to *NOT* get those cheap techs you've been ignoring. Farming the bottom of the tech tree reduces the total number of techs you don't have that they do have, which increases tech stealing time and reduces the value of trade routes. So, for example, if you're shooting for Artillery, you'll steal techs faster and get more beakers from trade routes if you ignore all those cheap 1 turn techs. Sounds counter-intuitive, but if you don't need a tech, don't get it until you need it.
* On the other hand, when you're just trying to catch up, for stealing bigger techs, *DO* farm the bottom of the tree, and try to finish up dependencies just in time for a steal. For example, if you're going warlike, and you stopped at Philosophy until someone entered the Renaissance, (T110 on Deity), then you should switch over to Drama & Poetry and Theology when you get your first Spy so you complete your first theft right when you finish Theology. This will get you Education as early as T125 without having to research it. Which isn't so bad. ;-)