According to my knowledge Polarizing filters are mainly used to make sky appear more bluish in the day sunlight which gives more dramatic effect to your pictures.

I don't have polarizing filters yet , But would like to know whether we can achieve same effect(making sky more blue like a polarizing effect) through post processing. If yes any link which explains it from scratch ?

I use GIMP and picasa (open source) in Ubuntu . Any easy solution using these Softwares ? or is it better to buy a polarizing filter itself !

I use Canon EOS 1000D , and mostly use 50mm 1.8 and 70-300sigma lens.