Throw It All Away Broken Remix (Mashup)
472MP3 download available: http://www.mediafire.com/?5zyut49mj5os3x5
The vocal theme of Shadow from Sonic Adventure 2, "Throw It All Away" by Everett Bradley... mixed with the Levitated Ruins level theme from Sonic and the Secret Rings, "High and Broken." Wow, that was a mouthful.
It takes a lot of work to make two songs work together... but do you want to know my secret?
I use a video editing program, like Adobe Premiere. The audio files are treated as "block" objects, and can be split, copied, and moved with ease. And on the 30 frames-per-second timescale, you're guaranteed to be able to line up songs on the beat. Audio editing programs like Audacity and Adobe Soundbooth don't give you that kind of flexibility.
I still use Audacity to do the tempo changing (pre-mix) and sound balancing (post-mix), but I choose to do the actual mix-and-matching in Adobe Premiere. That's my method.