So yes, mashups can be many things: two or more songs put together, and then you can make a video to accompany that; or they can be like a Google map with something else laid on top of it, or who knows what. Here we mean "a song set to the wrong (yet somehow so right) video, in this instance scenes from movies." We can get to the other mashup concepts later.
Soulya Boy - "Crank Dat" vs. Bambi
In which the steel drum-featuring megasmash is set to classic Disney animation, and the New York Times pays attention
Depeche Mode - "Suffer Well" vs. Tron
In which the melancholy UK synth-poppers provide an appropriate soundtrack to the 1982 film that I totally have to rent and watch again like right now.
Junior Boys - "In the Morning" vs. Bande a part
In which the melancholy US synth-poppers seem to inspire some coordinated dance steps in the classic Jean-Luc Godard film