On the subject of mash-ups, and considering all the crap that gets popular because of said mashups, I've decided to post something actually worth your time.
The Legion of DoomThey take good music, mix it with a little bit of electronic goodness and turn out something incredible. They have 3 songs available for download, one being a mashup of Dashboard Confessional - Screaming Infidelities and Brand New - The Quiet Things That No One Ever Knows, another being The Get Up Kids - I'm A Loner Dottie, A Rebel and Thursday - Understanding In A Car Crash, and the third being Atreyu - Ain't Love Grand and It Dies Today - Threnody For A Modern Romance.
Absolutely spectacular stuff. I highly suggest you check them out