And it is apparent to me that not many of our users here care to jailbreak for the useful tweaks that... jailbreaking brings.
Here's a few I use and some reasons why they are useful:
1) iFile: because it sucks not being able to manually manage files on the phone while I'm testing my apps.
2) SIManager: because I wanna make sure that if my iPhone goes down (out of battery), and I have to swap the SIM card into a dumbphone, my contacts should still be there.
3) MyWI: it's not for free tethering. It's for those moments when I need to have multiple devices on the same network, and I am suddenly without a Macbook. What is this for? Well, it's for things like... transferring files between my iPhone and another phone, for instance. Roundabout way, but better than not being able to do it at all.
4) OpenSSH + C/C++ compiler: allows me to develop simple applications... right on the phone.
Wait, what's #4 for? Well, when I lack a computer and an internet connection, and I want to run an algorithm fast, the simplest way for me is to use my phone to run that query. Call me a code junky or whatever, but this has helped me get out of pinches multiple times in the field. This is the only reason why I'd consider the iPhone a "smartphone".
I'd thought about writing an actual app with a GUI and what-nots for compiling and running C/C++ apps with GUIs on iOS multiple times, but it always ends up being a lot more work than it's worth, and Apple would most likely (101% likely) reject it if I ever try to submit it.