I'm going to kill two birds with one stone - one melody, two very different songs. The original melody comes from the parade march song found in Godfather II. The Monolith Deathcult and Immortal Technique both use it with awesome results that sound worlds apart.
The Monolith Deathcult - Demigod
This song is absolutely insane. Although death metal, its certainly not what you'd expect. Crazy pounding tribal drumming mixes with some pretty strange blasting. "Demigod" is both brutal and atmospheric, making for a very unexpected listen. The melody can be heard in three places: as part of a punishing rhythm guitar section, on ethereal synths and as part of a truly epic lead guitar passage.
Immortal Technique - Golpe De Estado
Man, this is fucking raw. On his mixtape The 3rd World, which is generally treated as his third album, Immortal Technique made his first (and currently only) song rapped entirely in Spanish. This has some guest spots, but Tech is really what makes this. He sounds raw, dangerous and fucking pissed off. The lyrics take aim at Latin governments. The melody works perfectly in a hip hop beat - its catchy and simple. As a side note, Immortal Technique isn't the only rapper to use this. When Jay-Z and Nas ended their infamous feud, they collaborated on a song that used the melody as the beat.