I've been known to get in the mood for these. Check out Stereogum and team9's annual Mysplice series, heres the most recent, Mysplice 4 (http://stereogum.com/archives/mp3/mysplice_4_106911.html
) and my favorite, 2008's MySplice III (http://stereogum.com/archives/mp3/stereogum-team9-present-mysplice-iii_041871.html
). There are two earlier installments that you can find somewhere on those pages. An interesting note, is that the series is targeted at indie music fans and usually feature songs as such, but then mash them with either that years pop hits Rihanna's Disturbia vs. Beck's Youthless) or older hits (Pina Colada vs. some Vampire Weekend song) or sometimes a new pop/hip-hop song vs and old hit (Kanye West's Love Lockdown vs. Gimme Shelter by The Rolling Stones), to stellar results. Seriously, this actally made a pretty decent song out of I Kissed A Girl as well as Taylor Swifts Love Story. I'd definately recommend them.
I like DJ Lobsterdust, made a pretty awesome version of 1979 by The Smashing Pumpkins vs. Jesse's Girl by whatever-his-fuckin-name-is.
Speaking of, 1979 has been the music to like 100 good mashups.