There is a simple yet addicting little game out there called Falling Sand Game: there are no rules or goals, but you have to be sure the imagination is your only one limit: rather than a game, i would call it simulation btw.
The first version i ever seen around was at the DOFI blog and since then, there have been other numerous variations on that theme such as this, this or this one too.
There is a fan-site too here and they have other numerous variations and add-ons to the original version, including a link to an SDL-based one, that’s a C++ port using the well-known SDL library: also, someone told me that there is a PalmOS version too.

At this point i was really expecting to see a [tag]PocketPC[/tag] version of that game, just because of the nature of the game itself: the game mechanics is coupled with the lowest human interaction i would expect to see in a game due to the fact that particles’ physics and properties do the rest, and this interaction is perfectly suited to a PocketPC’s stylus.
I ported the SDL-based version to the PocketPC platform, by using the cross-platform framework i’m currently developing: the screenshot on the right come from a win32-desktop build of pocketSand.
I plan to release it really soon, just the time to give it some tweaks and optimizations, mainly for the PPC build, and i really would love to have some feedback about it, such as how it performs on your device and/or if you are having problems just running it.

If you want to give pocketSand a try, stay tuned for updates!