The iPhone Dev wiki team ran the first 3rd party application on the iPhone. The program is a simple one, it spits out "Hello, World" when run; 'Hello, World' is typically one of the simpler things to program on most platforms and that it was chosen as the 1st app is unsurprising. Here's the notice they posted on their site:

After many, many hours of intense work from "Nightwatch", the first independent "Hello World"* application has been compiled and launched on the iPhone. This was made possible using the "ARM/Mach-O Toolchain", Nightwatch's "special project", that he has been working on so carefully over the past few weeks. Certain parts of the toolchain (such as the assembler) are being refined and tested and these will be released as soon as possible.

It should be noted that Nightwatch has been instrumental in creating these tools, working in near isolation to get them finished. Nightwatch was also responsible for the "jail exploit" that he developed from information he and other members of the the dev team discovered.

Please join us to thank Nightwatch, Tmiw, Darkten and Daeken for making this happen.

This is really big news; outside of Apple, there wasn't any known linker or assembler or ABI for Apple's Mach-O on the ARM processor platform (and a compiler is pretty useless without them). The toolchain that they built provides those missing tools, and now that it's confirmed to work, they can refine those tools and begin 3rd party development in earnest. W00tding, the iPhone goes up a level.