Matt Shapiro's 2011: Cinescape is one of the best movie mashups this year, blending a staggering 250 films into a cohesive whole. With the exception of the first clip from Sunset Boulevard (1950), all the films are from 2011 -- the full list can be found here.
Shapiro's continuity editing across totally different genres is what makes this particular mashup so compelling, but the guy's had some practice. Check out his trailers for 2006, 2007, 2008, 2009, 2010. Another outstanding cut worth watching is Gen I.'s fast-paced Filmography 2011.