midpssh & google maps are the most useful apps that are free. There are good pay for apps for folks that are in specific businesses - like the hot sauce trouble ticket system.
Personally - one of the best apps already comes with the phone - a back lit screen. One late evening I was in need of some heavy equipment and being on a construction site began going through several front end loaders, tractors, dozers looking for one that had the keys left in it. If it wasn't for the BRIGHT screen on my berry, I would have never found the keys for that Lull, and got our portable tower moved 58 feet south. Sometimes the phone's native ability is the application you're looking for. It's ability to serve as a flashlight has been extremely helpful & I think it's open source!
Sometimes I even call people on it.
It is easier to write an incorrect program than understand a correct one.