Tactile12000, from Tactile Pictures (www.tactile12000.com), is a standalone software emulation of a 2-turntable DJ rig. The software is a free download for Mac OS X and Windows, but Tactile Pictures happily accepts donations.
Tactile can play files in WAV, AIFF, MP3, and other QuickTime-supported formats. The program lets you crossfade, change tempo, and cue samples. To some extent, you can also execute turntable techniques such as backspin and scratch, though the results are somewhat primitive. Tactile is more toy than tool, but it is fun to play with and will give you some feeling for the art of the DJ.
After you've loaded Tactile's playlist with audio files, you slap individual files onto the turntables and manipulate Tactile's controls during playback. Separate crossfaders for the cue and main outputs allow you to set up one turntable while playing the other. File selection and playlist maintenance are fast and simple. In addition, you can use Tactile's AutoMix mode to sequence the entire playlist, crossfading between tracks as it goes.
You access Tactile's documentation through a multipage help window; rollover text also describes the function of each control. The program has computer-keyboard shortcuts for each of its many controls, and the shortcuts are displayed on the graphical user interface when the help window's keyboard-shortcuts page is displayed. (Printing a screen shot of the shortcuts is very helpful for getting the gist of Tactile.) Some functions adapt well to keyboard shortcuts, whereas the mouse is the best choice for others. MIDI would be a preferable alternative but unfortunately is not an option.
Tactile's audio output is directed to Core Audio's default output in Mac OS X, but under Windows, you can assign it to any sound card with a DirectX driver. In the latter case, the main and cue outputs can be directed to different sound cards for more-realistic headphone cueing. When only one output is connected and cueing is turned on, the cue output is directed to the left channel and the main is directed to the right. On the Mac, an audio-routing utility such as Cycling '74 Soundflower can route Tactile's output to another audio application for recording as well as for splitting the cue and main outputs. (Tactile has no provision for recording its output.)
If you're feeling tactile, grab the download and start spinning. Even if you're not, the AutoMix function is well worth the free download.