Mashups have gotten a bad rap and with good reason. Every Tom, Dick and Harry tries to make a mashup but every now and then a mix comes along worth the listen. The three songs below are my favorite mashups at the moment, my favorite being Poppa's Teardrop.
Tone-E - Golden Ghostwriter (Jurassic 5 vs. RJD2)
DJ KPAC - Poppa's Teardrop (The Notorious B.I.G. vs. Massive Attack)
The Hood Internet - Freeze, Barbra (Young Dro vs. Duck Sauce)
The following two songs are also great but then again, anything with Passion Pit or Phoenix usually is.
The White Panda - What You Know About That (T.I. vs. Passion Pit)
Basic Physics - Stuntin With A Milli (Lil Wayne vs. Phoenix vs. Pretty Lights)