I pared quite a few of these down.  Originally I just ripped these off Novel Guide, but I decided that the symbolisms didn't fit, were dumb, or were stretched just a little too far to be realistic.  So, these are more of my opinion, with a little help from a book.  See Credits.

Alcohol
Animalism
Bank Notes
Battle of the Cowshed
Battle of the Windmill
"Beasts of England"
Farmhouse
Manor Farm
Rebellion against Jones
Windmill

Alcohol: Orwell uses beer to represent the "Old" Russia.  He first notes that the reason Jones lost control of the farm and began being cruel to the animals was because of alcohol.  It symbolizes, more than anything, a corrupt government— a government drunk on prosperity (a prosperity which never trickles down to the common animal).  But it's eventually this drunkenness which ruins and leads to the inevitable collapse of this system.  Jones lost power over the animals when he became drunk and lazy; even Napoleon will eventually be overthrown because of the alcohol he intakes.  Orwell alludes to this near the end of the book when he says that in generations to come there will be still more uprisings.   "Some day it was coming: it might not be soon, it might not be within the lifetime of any animal now living, but still it was coming."
     Alcohol was originally seen as a grave evil of the new regime.  Old Major repeatedly warns the animals against taking on Man's ways, but his concerns are not heeded.  Really it was the issue of alcohol that made many of the animals suspicious of the pigs.  Thus, Napoleon had Squealer change the commandments.    
     It's interesting that even today many of the Russian leaders have a drinking habit.

Animalism: Communism/Socialism.

Bank Notes:  Well, it seems that Nazi Germany learned from WWI that fighting a two-front war was more than they could handle, it having led to their defeat.  So, Hitler wanted a guarantee from the Soviet Union that the two countries would not fight each other, leaving Hitler free to concentrate on western-european threats.  Any disputes between the two countries were to be solved in a friendly-fashion.  A representative from Germany, a guy named Ribbentrop, met a rep from the Soviet Union, Molotov, on August 19, 1939.  They first signed an economic agreement: Germany would exchange manufactured goods for Soviet raw materials.  Four days later, on the 23rd, they met again to sign a non-aggression pact which was to last for ten years.  Germany kept its end of the bargain for only two years, having intended all along for this pact to be temporary.  Hitler had been simply waiting for an opportune time to invade the Soviet Union.

In Animal Farm, Napoleon sells a pile of timber to Frederick and demands "real five-pound notes."  The notes are paid and the timber is quickly carted away.  Napoleon announces that the sum will pay for the machinery for the windmill.  Supposing that Animal Farm represents the Soviet Union and Frederick represents Germany, the timber (raw materials) are being exchanged for windmill machinery (finished products).  But then three days later Whymper comes in to tell Napoleon that the notes were fakes.  Frederick had stabbed Napoleon in the back.  The forged notes refer to Germany's turnabout on the pact.  As Germany didn't keep up the agreement and attacked the Soviet Union two years after the signing, Frederick didn't keep the agreement and paid in fake notes.  Immediately following the timber deal, men attack the farm, too, which could also symbolize Germany's invasion of the Soviet Union.

Battle of the Cowshed: the anti-Communist invasion of Russia in 1918-1919.

Battle of the Windmill: the German invasion of 1941.  Notice that the Germans invaded two years after signing a non-aggression pact with the Soviet Union.  More on this here.

"Beasts of England": the Communist anthem "l'Internationale."

Farmhouse: The Jones' farmhouse represents in many ways the very place where greed and lust dominate.  Unlike the barn, which is the fortress of the common man, the genuine concept of socialism, the farmhouse, where Napoleon and the pigs take over, symbolizes the Kremlin.  Even today the Kremlin is an important place to Russian leaders, who, instead of embracing Marxism, have created their own distorted view of communism and have shoved it down their peoples' (animals') throats.

Manor Farm: Obviously it represents the Soviet Union.  But the Soviet Union was supposed to be a worker's paradise, like the pigs promise.

The Rebellion against Jones: the Bolshevik Revolution

Windmill: The Five-Year Plans which Stalin designed to industrialize the Soviet Union.  Ironically, Orwell didn't write a final collapse of the windmill, which would perhaps have symbolized the U.S.S.R.'s failure in the cold war. In 1990, the Russian Communist government collapsed due to lack of funds. Of course Orwell could not have known this, although he did forecast a future rebellion.

 

Credits:

Novel Guide

"Student Companion to George Orwell."  By Mitzi Brunsdale.