Where can I download the flight plan data around the world?
For historical FAA IFR flight plan data, you can get it from www.flightaware.com.  They show five minute delayed, and with a free registered account you can get access to the last 90 days worth of data.  You can pay for data older than that and I believe in different data formats. 

I'm not aware of a similar source for other parts of the world.