Program: Stepmania 3.9 Final, 3.9+ (currently being developed), 4.0 CVS/SVN (again, in development)
License: Open Source, Creative Commons
Description: Dance Dance Revolution simulator, playable with both a dance pad or keyboard.
Website: http://www.stepmania.com/
Other: The only data that I am aware of that is not stored in the original program folder is the machine settings (key bindings, top scores, profiles, etc). That is, by default, saved in the My Documents folder. The head programmer for the project (Chris Danford) was considering changing the location of the machine settings to the program folder in a thread I created, mentioning this (http://www.stepmania.com/forums/showthread.php?t=12773), but I figure that integrating SM into the portable apps menu would also be quite nice.
as soon as it meets the runtime requirements.
*Edit*
I need a few things first. I never saw the Creative Commons license before, can you please post a link to it?
Also, what kind of dancepad is the site talking about? A DDR mat that's been hacked into the computer?
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Further investigation shows it is actually an MIT license. The source code for Stepmania has been modified multiple times to create commercial programs (In The Groove, Pump It Up: Pro, etc.) and the developers encourage people to modify the game and redistribute it. (SM 3.9+, KuriMix, mDaWg's ITG, MungyoDance, etc.)
http://sourceforge.net/projects/stepmania/
You can buy USB dance pads, or dance pads for other systems (XBOX, PS2 etc.) and use adapters to connect them via USB.
I personally have a Cobalt Flux dance pad, which has a control box which allows it to connect to USB, PS2 or XBOX.
http://www.cobaltflux.com/
EDIT:
Also, if memory serves, the developers recommend using Visual Studio 2003 or 2005 (I think 2003) to compile, newer versions such as 2008 have a lot of trouble compiling the code, it's rather delicate...
Will this be portable or not?
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after all the stress I'm getting, I'm probably not gonna do any apps for a while, just update.
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It's not whether or not it can be done, it CAN be done, there is no question about it, it's whether or not anyone is willing to do it. And I can fully understand if no one wants to take up the task of updating this every release until 4.0 Final is out. This might even be done by the developers soon enough, to be honest, but 3.9 is not portable either, and will never be unless someone here does it.
tl;dr - It can be done, but it is a bit of a task to update since there are so many different versions out there.