iPhone hacking efforts have started to bring some rudimentary tools for developers to create their own applications on the iPhone. Unfortunately, the process of developing applications or even installing 3rd party applications remains cumbersome to most casual users.
One example application circulating today is Mobile Terminal (video) which allows you to have shell access on your iPhone.
A TUAW blogger writes about her experience playing around with the unofficial iPhone developer tools.
Again, while all these efforts are promising and the tools should only get easier, until Apple comes out with their own official developer's kit, we won't see the true potential for 3rd party applications on the iPhone.