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www.mixmeister.com

MixMeister Technology
MixMeister Technology creates innovative software products for playing, producing and performing music mixes. If you're new to digital music mixing, MixMeister software will let you unlock your "inner DJ." If you’re an experienced music mixer, you’ll find that MixMeister takes your skills to new heights with advanced tools found nowhere else.
 
Apart from Ableton Live (which many would argue is not really a DJing tool) Traktor DJ Studio seems to be the best bet. I use it myself but still slowly getting to grips with it.

Not cheap. Demo available.

http://www.native-instruments.com/index.php?id=traktor3_us


Ars review of various DJing apps

For my own needs, I like Traktor DJ Studio and unfortunately for Mac users, it's really the only good choice for serious DJing software. I think that while some potential bedroom DJs might be turned off by its austere design, it provides a ton of growing room and creative mixing potential.

http://arstechnica.com/reviews/apps/dj-software.ars
 
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