According to the Concise Oxford Dictionary, a flash mob is a public gathering of complete strangers, organised via the internet or mobile phone, who perform a pointless act and then disperse again.
Of course, some acts are more sophisticated than others, such as impromptu musicals and dance, so one would expect a certain amount of rehearsal time by these 'complete strangers'. By definition, they shouldn't be considered as flash mobs but more as unannounced performances in public spaces.
Still, seemingly spontaneous displays of artistic merit are the most fun to watch. They are always usually captured on film, sometimes of a high professional standard.
Mashable listed its '15 Fab Flash Mob Videos on YouTube' back in June 2010.
My list is different. Here are some of the best.
From the Eurovision Song Contest in late May 2010, dance simultaneously performed throughout Europe to Madcon, Glow
Ohio Union at Ohio State University on 3 May 2010, dance to Glee soundtrack, Don't Stop Believing (itself a cover of Journey's original song)
Antwerp Central railway station - dance to Do-Re-Mi in March 2009
Bondi Beach, Sydney - dance to Ben Lee, That's the way I like it, in November 2009
Improv Everywhere's Food Court Musical in March 2008
Opera (yes, really)
Opera Company of Philadelphia 'Brindisi' from Verdi's La Traviata at Reading Terminal Market on 24 April 2010
Opera Company of Philadelphia 'Hallelujah chorus' from Handel's Messiah at Macy's in Center City Philadelphia on 30 October 2010 as part of the Knight Foundation's 'Random Act of Culture'
Organised by Alphabet Photography, performed by Chorus Niagara at Seaway Mall on 13 November 2010