Mashups are relatively facile to do, very easy to get wrong — and exceedingly hard to go beyond a mere "get right", transcending the apexes and hitting some sort of cross-fusional aspect… like mating a whale shark with a giraffe and having their babies look beautiful. From the rhythmic structure (often 4/4 time with 8-bar chunks) necessitating tempo-matching, to hoping the riff that comes in at one point of Track A accentuates the vocal in Track B, there's a lot to match, and yet… pick an ideal pairing, breed 'em, and you've got magic. It's even better when they have videos!

Here are 2 of my faves:
 

"Bruised Water" - Natasha Bedingfield vs. Chicane

Maire Brennan's ethereal vocals in the latter ideally complement Bedingfield's sassy street-smartness in the former. I admit I never heard the original "I Bruise Easily", but one of the true signs of a great mashup is when you hear it, and believe that to be the original work. Such is the case here, where classic trance arps play off swollen lyrics, and even the intro of the plucky chuffing @ 1:30 insistently declares, "THINGS CAN ONLY GET BETTER FROM HERE!" And that they do. You have to hear the breakdown section for yourself; it goes together as well as violet and indigo on the rainbow, only a lot more colorful. Bonus points for the video being well-edited between the two sources as well! :)

 
"My Suffering Love (CjR Mix)" - Justin Timberlake vs. David West

I think David West (go there and download some free mixes!) is such a great electronic musician, and while I'm not as familiar with the repertoire of Timberlake, I find him to be a great singer. Since the David track doesn't have a vid, this video is all Justin, but his vocals laid over the gatey prog arpeggios will have you both tapping your feet and headbangin'… peacefully. Again, a triumphant breakdown section, and the slow-mo violins flying forth @ 3:16 along with the triangle wavesque repeated riff are a beautiful way to make an impression on your psyche. Whoooosh. Indelibly catchy — in a better alternate universe, this is what radio stations are playing.