This has already been posted widely but in case you didn't see it this is DJ Earworm's United States of Pop 2008:
This mashup combines samples from each of the Billboard Top 25 songs of 2008. Earworm made the video as well. DJ Earworm also created a mashup combining the Billboard Top 25 songs of 2007 last year:
Both of these and many other excellent mashups can be downloaded from DJ Earworm's site.
Earworm is one of the most talented mashup artists out there. His production skills are incredible. Earworm is also the author of a mashup instruction book which is quite good although sadly not Mac compatible (he uses Sony's Acid Pro). DJ Earworm is best known for creating mashups with a large amount of samples all melded together seamlessly. However unlike other mashup artists, such as Girl Talk, that use tons of samples in a sort of frenetic collage, Earworm's mixes sound much more like recognizable songs with verses, choruses, and lyrics from multiple sources that are often tied to a common theme. For more about DJ Earworm's style check out this article from the MIT Technology Review which also features a cool "make your own mashup" tool.