I've always wanted to turn an old PC into a networked jukebox. Usually, I leave the current OS on it and use it for development testing. A firewall would be good, but don't forget physical security. Many web cams come with activity monitor software...
I complain about the very same thing. There hasn't been a lot of development effort for building anything cross-platform. Out of fifty LISP implementations I looked at, only a few were cross-compatible. Hardly any computer games are multiplatform. I build my own software and release it on Mac, Linux, and Windows whenever possible. It's been really easy starting with a common code base (C/C++/Java/whatever) and creating the GUI using native tools: Xcode, Visual Studio, Glade. The GUIs end up looking good, are easy to make (if a little more trouble to sync between the OSs), and I don't have to spend hours scouring the 'Net for information on GUIs across several operating systems. It's becoming alarmingly common for Google to return no search results about some computing topics and programming errors. If I can't Google it, I can't fix it.
One of my biggest problems has been incomplete programming language support. Why should I learn wxWidgets if wxRuby is half-done? The documentation for RubyCocoa is even less substantial. I can't begin coding unless I understand a minimum of how the various GUI objects interact. I finally understand all those dumb underscores in RubyCocoa methods. Objective C is odd, but consistent.
Something like a Swing but C based would be nice, but it's a lot of trouble to wrap native methods (menubars, system trays, pop-ups, etc.) and not that rewarding. It's easier to extend a codebase using native tools for the interface.
This app has a ridiculous amount of ports in assorted languages and toolkits. I've done it with Perl, Python, Java, Lua, Ruby, C, C++, and Objective C; on Tkinter, wxWidgets, Gtk, Qt, Glade, Cocoa, Windows forms, Swing, and HTML/Rails. Ultimately, I just use the command line interface and simple Mac/Windows/Linux GUIs. The screenshots are Cocoa, because it naturally looks the best!