Sony Acid Xpress
Free

The free, cut down version of Acid Pro, will give you a feel for the main program. I'm not sure on the functionality - but if you're looking for a free sequencer, this might do the trick.
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Reaper
Free

Looks like Acid, behaves like acid, but is free. It's a multitrack sequencer with the only downside being a lack of beatmatching tools as far as I am aware.
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Audacity
Free

Essentially a free 'Soundforge' Audacity is open source software for recording and editing sounds. It is available for Mac OS X, Microsoft Windows, GNU/Linux, and other operating systems.
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Rapid Evolution
Free

Rapid Evolution is an advanced harmonic music library tool for dj's, designed to be used while you mix. It helps profile your songs by saving important properties, such as the bpm, key, time signature, styles, comments, etc. It lets you save comments and information about each individual mix you come up with.
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Tap Tempo
Free

Have you ever wanted to know exactly what BPM (Tempo) a song was at? Well, there's a better way then guessing, and TapTempo is the tool to do it. Simply run TapTempo, play the song, and press the spacebar on every beat; after a few seconds, you'll see the tempo (along with other info) displayed!
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Mixmeister - BPM Analyzer
Free

MixMeister BPM Analyzer is a free program you can use to determine the exact BPM (beats per minute) of any song.
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Mixcraft
$39.95

Mixcraft™ allows you to record and mix music. It’s designed to be fast and easy. Cut, paste, edit and shape your sounds the way you want! Use effects or install a variety of DirectX or VSTs This flexible software can be used in many ways.
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Sony Acid Pro
$374.96 (from)

One of the most popular sequencers with mashup artists, this sequencer from Sony can do plenty, including beatmatch and key changing utilities.
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Mixmeister (Express, Studio & Fusion)
$49.95 - $329.95

MixMeister software isn't quite like anything else on the market. It's designed for people who care enough about their music to refine a mix until it’s perfect, rather than throw everything together on the fly and hope that it sounds OK. You won't be limited by on-screen simulations of turntables or CD players, either -- you can "tweak" any part of your mix creation in a timeline instead. When you've got the mix just the way you want it, you can play it, burn it to CD, stream it over the Web, or reshape it in real-time for live performances.
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tONaRT
(contact)

tONaRT key detection automatically determines the key signature of any audio input signal, either monophonic or polyphonic. As a special feature, tONaRT also detects the standard (concert) pitch of the input signal.
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Ableton Live
$499 (d/load) $599 (boxed)

Award-winning music creation, production and performance platform for Mac OS and Windows.
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SoundForge
$299.96

Sound Forge software is everything you need to analyze, record, and edit audio, produce music loops, digitize and clean-up old recordings, model acoustic environments, create streaming media, and master replication-ready CDs.
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Info...

This is a list of software that you can use for mashups. There are sequencers, wave editors and other plug-ins as well as handy tools like BPM analyzers and so on.

If you know of a product that should be added to this list - please get in touch. There is an email address on the home page.