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Required Tech: Banking
Effect: +1 Free Specialist per City, No Foreign Trade Routes, Foreign Corporations have no effect
Use: Getting free additional GP points in Ironman/ffa and Renaissance to Future start games. Increasing research in combination with Representation. Speeding up start with additional Hammers from free engineers in Renaissance to Modern start games.
Mercantilism provides you with a free Specialist in all your cities while having the (for ffa diplo games severe) backlash of disabling Foreign Trade Routes. If you play an ffa diplo game and have open borders with several players, Mercantilism is no option, because the Commerce you loose due to that, cannot be compensated by the Specialists. This is especially the case, when you play on a map with continents, where you have intercontinental Trade Routes, that provide a heavy additional boost.
Also Free Market gets available at about the same time on the Economy branch, providing you with another of those oversea trade routes instead of loosing them all.
In always war type games you don't have foreign trade routes anyway, so this is no problem, which makes Mercantilism an interesting option. While you only have Free Market as different option, Mercantilism will always provide a better bonus.
If you play later Era start games where Mercantilism is available from the beginning, it allows you to work an Engineer in each city from the very beginning, giving you two additional hammers (you have a Forge in your cities when planting from Renaissance upwards), which is a pretty nice boost for your buildup.
If you run Mercantilism together with Representation (the case with specialist economies in ffa/Ironman and Industrial Teamers) and work Merchants/Scientists each specialist provides money/science equal to 6 Commerce. This is quite considerable and e.g. the way to get to Communism in Industrial Teamers.
Finally the simplest use of Mercantilism is getting additional GP points from specialists if you want to push out GP. This is valid in any type of game.
[U][B][SIZE="3"]Free Market[/SIZE][/B][/U]
Required Tech: Economics
Effect: +1 Trade Route per City, -25% Maintenance Costs from Corporations
Use: Increasing base Commerce from Trade Routes and increasing effect of Corporations in ffa/Ironman.
Free Market has a limited range of uses, but can be very powerful at those. The additional Trade Route equals about 8 Commerce per city, if you run international intercontinental trade routes. And you don't have to do anything for that. On the other hand, it only helps a lot in that scenario. With international, but no intercontinental Trade Routes it's still about 4-5 Commerce. With always war it's virtually useless.
The other use for Free Market comes up when having Corporations. If you have two or even three Corporations spread to all your cities, those 25% save you a lot of money on maintenance. There is just one, unfortunately rather big problem: At that point of the game, you will have serious health issues from building Factories and Coal Plants, so you might be forced to run Environmentalism. If that is the case, try to solve those health problems as fast as possible and go back to Free Market.
[U][B][SIZE="3"]State Property[/SIZE][/B][/U]
Required Tech: Communism
Effect: No Maintenance Costs from Distance to Palace, Corporations have no Effect, +10%
in all cities, +1
from Workshop and WatermillUse: Workshops Economies in Industrial to Future start games.
State Property is pretty much useless for Ironman/ffa games, because you shut down your Corporations with it, which are far too strong to do so. Also you don't run massive Workshops in those games, and that's where Communism really kicks in.
The single use of State Property is Workshop economies as already mentioned. To those it's mandatory, because only the additional food from Communism lets you grow your cities big enough.
[U][B][SIZE="3"]Environmentalism[/SIZE][/B][/U]
Required Tech: Medicine
Effect: +6
in all Cities, +2 from Public Transportation, +2 
from Windmills, Forest Preserves, +25% Maintenance Costs from CorporationsUse: Preventing your cities from starving whenever available and necessary; With National Park and Forest Preserves in One City Challenge games.
Except form the use in OCC, Environmentalism is pretty much an emergency Civic. It doesn´t do anything positive, except solving possible health problems. However those happen to appear quite often. When you build Factories and Coal Plants in an ffa/Ironman game, researching Medicine for Environmentalism to counter those right away is a very valid option. (Especially because it's also the way for Sushi&co.)
In Future and also some Modern start games, you will have to adopt Environmentalism from the start, to prevent your cities from starving right away, because you already have five population and factories in your cities, but not any health resource yet.
[B][U]See also:[/U][/B]
[URL="http://apolyton.net/entry.php/17-Civics-in-Focus-I-Government-%28Civ4%29"]Part I: Government[/URL]
[URL="http://apolyton.net/entry.php/18-Civics-in-Focus-Part-II-Legal-%28Civ4%29"]Part II: Legal[/URL]
[URL="http://apolyton.net/entry.php/23-Civics-in-Focus-Part-III-Labor"]Part III: Labor[/URL]