Analog Simulation Tool
We will probably want to try simulating different parts of Upside, particularly the power electronics along with the control circuitry and whatever other analog circuits we have. I recommend LTspice for this, from Linear Technologies (soon to be part of Analog Devices). It's a very capable SPICE tool used in industry, and is also a free download (though it's not FLOSS). There are Windows and OS X versions available, but I run it in Wine.
Another benefit is that it comes with a big selection of component models, including ones for switching regulators Linear Technologies makes. If we want to look at any of these, this is a good way to check it out.
Here's a link to the Wikipedia page, which has a couple screenshots and more of a blurb, and links to the download page: https://en.wikipedia.org/wiki/LTspice
There's also Ngspice, which is FLOSS: http://ngspice.sourceforge.net/ I haven't used it personally, but would be interested to hear if anyone has.