Well, i've been searching for a good two hours now and for the sake of me i can't find a way to make a link to another post, so here is my brief list of ideas
iSync without .Mac between two connected Macs, but i think it should evolve step by step, the first synchronization could be Finder folders, then AddressBook, later Safari bookmarks, then iCal, Mail, iTunes, iPhoto...
Another idea is a bluetooth networker, that would allow me to share my internet connection with another bluetooth enabled Mac, or access files from the network menu...
Another (simplier) idea is to develop an app that changes the system volume according to the surrounding noise, like some Alpine auto-stereos used to do.
In the same direction, an screensaver that could only be deactivated by voice recognition.
In the security area, another idea i've have is an app that would ask for your administrator password every once in a while, perhaps every two days or something not-so annoying, for those users who use their Mac in Automatic Login fashion or for those that never shutdown it, i know there used to be a similar app for the Palm platform and i loved it.
Another security idea could be an app that would start an alarm or scream really loud upon waking up in case anybody moved your Mac while in sleep mode...
In the movement sensor area, it would be awesome to develop a virtual joystick that could work by actually moving the whole Mac (it would be useful mainly for laptops).
Another idea i have is to have a force-feedback simulator that could be developed by spinning up both the cd and the hard drive while emitting low frequency tones from the speakers, it could be useful for those times when you can't use sound nor visuals(while giving a lecture, for example) but the system needs to alert you. (an example could be an iCal alarm, or an incoming call in your bluetooth phone stored in your backpack, etc.)
In the sound area, a ultrasonic emitter could be useful if you're using your 'book in a mosquito-infested area.
In the mosquito-infested theme, i could definitely use a menu bar icon scroller for all those menu items that get too crowded in my 12" screen.
Another way to save menu space in the 12", could be an app that would monitor the system and light up the numlock led (or the sleep light in the 'books) whenever some condition happens, being it a new mail, an unanswered adium message, extreme temperature in any sensor, SMART alert, low battery, iCal alarm, low/lost airport signal, it could be made to have up to three conditions with slow blink, fast blink or fully lighted.
In the temperature area, an app that would let you define the temperature at which the fan starts would be warm-welcomed i think (but obviously would need a full disclaimer).
Another app i can think of is a screen low brightness fine-adjuster, because i find that sometimes the lowest lighted setting for my iBook is still too bright, for example to read in the bed with your beloved one by your side trying to sleep.
A very useful one would be an auto scroller for those times when you are reading for example a very long post in Safari (like this one! ), or a long text in textedit and you don't want to click every few seconds, it could be activated by looking if the window below the mouse has a scrollbar, and if it haves, defining a scroll startup delay and a scroll rate (it could be made even better by defining the scroll speed depending on both the distance from the mouse to the title of the window and the size of the font).
Another useful app for us 'book owners would be one that would allow us to use the trackpad as an alternate input device like the wacom tablets by pressing a control key or while using an external mouse.
These are only some of the most basic ideas for new apps, please don't get me started! And of course all this ideas are free both like in beer as in speech, but a nice mention somewhere deep in the documentation would be most welcomed.
As someone used to say: "live long and prosper".
Leo