Years ago, Acid (I forget who made it) used to have great features for beatmatching tracks and pitch shifting. It wasn't the best DAW by any means but it seemed to have been designed for making mashups.
I have an unusual amount of knowledge about Acid since a friend of mine originally created it (he left Sonic Foundry to work on audio software at Apple the last time I talked to him) and I used it to produce two albums back in the day.