Using Google Trends, you can compare the evolution of the popularity of different products or brands:
Yahoo Mail vs Gmail vs Hotmail -> Winner: Hotmail, second place: Gmail
XBox vs PSP -> Winner: XBox
YouTube vs Google Video -> Winner: Google Video (but YouTube has a very fast rise)
Winamp, Real Player, Windows Media Player, iTunes -> Winner: Winamp, second place: WMP
Firefox, Opera, Internet Explorer, Safari -> Winner: Fiefox, second place: Opera
Digg vs Slashdot -> Winner: Slashdot (Digg has a good evolution and the two news sites are almost equal)
RSS vs Atom -> Winner: RSS
Apple, Linux, Windows -> Winner: Windows, followed by Linux
Larry Page, Sergey Brin, Bill Gates, Steve Ballmer -> Winner: Bill Gates (by far)
Wikipedia vs Britannica -> Winner: Wikipedia