EPx summer of code

Time to have that glowing enthusiasm for new things, as children do! I have committed myself to learn a bunch of things during the next few months: Ruby on Rails, Cocoa, OpenGL, and recall Qt. (I'd also like to try Lazarus and Harbour to honor my Delphi and Clipper days, but those ones will have to wait till I win the lottery and programming becomes a just-for-fun activity).

Since the only way to truly learn a programming language is to write real, useful applications in it, I need ideas for such applications, given the technologies I am trying to get used to. My current choices are:

* For Ruby on Rails, a personal accounting/budget application. Since I've given up on booking all my personal finances in detailed double-entry ledger (it takes too much time and does not create new money after all), I would still like to have an automated way to control my finances. It is boring stuff, and no chance of being a new hit in the market, but at least will be useful for myself.

* For OpenGL, I plan to write something like a crude traffic simulator, for cars or maybe for trains. Given a coordinate description of tracks or streets, plot the map and run a simulation on it. Make a railway simulator would be a bit more difficult, but much more fun for myself, since I love trains. In the other hand, in the case of a car traffic simulator, it could be fun to add scriptable driver behavior etc.

* For Cocoa and Qt, my current plan is to write a simple financial calculator, not like HP-12C, but more like this one. Once the desktop versions are working, a lot of derivatives can be made, in order to learn Cocoa on iPhone, Qt on Maemo, Mac OS X widget in Objective C, and so on.

I am sure that there are better applications, useful for many other people people besides me, waiting to be written, but I am a bit short of ideas. Anyone with good suggestions is invited to comment.