I am sure I've posted this before. But it is timely.
Stratoblogster posted a link to Axis of Awesomeness proving that a third of all popular songs use the same I-V-vi-IV progression.
Another third is all on Pachelbel's Canon, which is D A B F# G D G A. (Thanks guy in vid.) Goes to D A Bm F#m G D G A for chords, to I V vi iii IV I IV V. A big more complex, so it's good to only have to learn one five-chord progression. B)