I've got two, although I'm sure I'm in the minority of needing these...
1) An app which will easily let you enable/disable email accounts on the iPhone.
Right now I have 5 email accounts set up on my phone, 2 of them are my "main" ones, the other 3 I just use occasionally (usually during the day when I can't check mail at work and I'm expecting something) - when I have to use them I go to the springboard page with 'Settings, then Mail, then individually open each mail account, choose on/off, then go back and do it for the other accounts. I think it would be great if someone created an app which would list all of the mail accounts one one screen and let you do a mass enable/disable.
2) Another nice app would be to allow you to set your sounds based on the time of day. For example 7:00AM-11:00PM Monday-Friday I want Vibrate to be on, but when I go to sleep I want vibrate to be off completely. (I keep the iPhone on my nightstand and if I get an email at 3AM I don't need the vibration going off). Right now what I do is go into Settings> Sounds > Vibrate and toggle it each night (when I remember) and then I have to un-do it in the morning.
Again, I don't know if these really have mass appeal, but if theres a developer looking to come up with something new I'd definitely use them!