One of my primary reasons for not being too in to the iPhone/iPod Touch third party app scene is that I couldn’t see one single application that I would really need (or want enough to go through the trouble).
Finally the game has changed, no there are two new apps have been developed that make 3rd party apps worthwhile on the iPhone (in my opinion).
First, MMS for the iPhone. This is very early in development but, you can send MMS messages to other phones. Currently you can only send MMS messages, you cannot receive them, you can also only send images, movies aren’t supported for now. But, the developers say that the limitations will be addressed in future releases.
Secondly, a VOIP application. This is in much earlier development stages than the MMS app, but it does work. The application is actually a port of the Nintendo DS-based SIP app. The developers have been able to initiate calls and receive calls with the client, but it seems to be only a command line application at this point.
These are the types of applications that (if they were in the later stages of development) would probably persuade me to jailbreak my iPhone.