I've known about and enjoyed the YouTube mashup of clips from NBC's The Office with the song SexyBack. The person that made the mashup obviously spent a lot of time finding clips that fit the lyrics and also the major changes in the song and made it look really synchronized. Then today I saw a link for the Mr. Roboto mashup and found that it was also an excellently edited mashup. So I dug a little more. In all I found four really good Office mashups, and here they are, all in one place:

SexyBack - the first one I was aware of

Domo Arigato, Mr. Scott-o - has a surprising amount of relevant clips...Dwight on the recorder is priceless

It's Getting Hot In Here - I like how this one has several spots where it looks like characters are singing lyrics

Scranton Party - this one's a bit repetitive because the source material is limited, but it's still a good one and short as well