Google Trends tracks popularity over time by region and language of search terms entered in Google.
A few preliminary results are interesting:
* Firefox has leveled off since rising rapidly throughout 2004. It is especially popular in Germany
* Del.icio.us peaked at the end of 2005 but may be recovering. It's especially popular in Southeast Asia.
Result timelines may include landmarks dates: For example:
A After 125 years, ukulele still keeps 'em smiling
Arizona Republic - Apr 21 2004
B Hawaiian musician takes ukulele to stardom
KVUE (subscription) - Aug 26 2004
C Warren Buffett plays ukulele, answers tough questions in packed Berkshire weekend
San Diego Union Tribune - May 2 2005
D Hawaiian ukulele star rockets up charts
Wilkes Barre Times-Leader - Jun 13 2005
E Ukulele shop grew into historic status
Honolulu Star-Bulletin - Jul 17 2005
F Ukulele kids rock in class and after school, travelling the world to perform
Brandon Sun - Feb 18 2006
You can also compare trends. For example:
Yahoo consistly scores higher than Google (because searchers are already at Google?).
Japanese art consistently scores higher than Chinese art (but interest in both is declining, if search queries are an indication).