The Polish Home Army and Underground State possessed arguably the largest active resistance movement in WWII both in terms of total numbers(roughly the size of the entire Soviet resistance to the Nazis, as well as in per capita terms.
It was pretty nasty of the Soviets to sit back and let the Nazis/Polish Resistance to chew each other apart. Postwar realpolitik and military economy of effort and all that, but not a nice thing to do for folks supposed on the "same side".
And it didn't end with WWII:
http://en.wikipedia.org/wiki/Cursed_soldiers
The Poles continued to actively fight against the tide of Communist totalitarianism post war.
Witold Pilecki fought in the Warsaw Uprising, causing the Germans considerable trouble:
http://en.wikipedia.org/wiki/Witold_Pilecki
He's the stud who came up with the plan to infil and recce Auschwitz from the INSIDE:
http://en.wikipedia.org/wiki/Witold's_Report
The Communists/Soviets had him executed post war.
He was rehabilitated by a free Poland in 1990, and recognised retrospectively since.
I've got a few books about the Polish resistance in WWII:
The Polish Underground 1939-1947 by David Williamson
Station 43 SOE's Polish Section by Ian Valentine
Thirteen: My 13 years in Polish SMU GROM has been translated into English.
After reading what they've done then and more recently, I'm glad the Poles are officially on our side.