actually, here's the list of the top ten largest concerts ever:

10) Blockbuster Rockfest 1997 - June 21, 1997
385,000 people

9) Woodstock 1969
400,000 people

8) Toronto SARS benefit - July 30, 2003
450,000 people

7) Simon and Garfunkel in Central Park - September 19, 1981
500,000 people

6) Isle of Wight Festival - 1970
600,000 people
*this was Jimi Hendrix's last performance, as he died shortly afterwards

5) Summer Jam at Watkins Glen - July 28, 1973
600,000 people

4) Steve Wozniak's 1983 U.S. Festival - May 28-30, 1983
670,000 people

3) Garth Brooks in Central Park - August 7, 1997
750,000 people

2) New York Philharmonic in Central Park - July 5, 1986
800,000 people

and the largest concert ever was...

1) Rod Stewart at Copacabana Beach - December 31, 1994
3.5 million people

and here's where i got my info:
AskMen.com - Woodstock