There are loads of programs out there that will do this. If you are unhappy with iPhoto, you might start with Google's Picassa which is also free. On the paid side, Pixelmator has a pretty good auto-enhance feature as well as a whole slew of other features (roughly equivalent to Adobe Elements, but much easier to use and in some areas functionally superior), and can be purchased for about $50 if you use a cupon code (sometimes even cheaper).
I've never cared much for Nikon's software. Nikon makes great cameras, but they usually outsource their software to third parties. Capture NX Pro, for instance, is made by NIK Software. NIK happens to make really good stuff (of course that's probably why capture NX costs $200). On the lower end, I strongly doubt that Nikon is providing any proprietary knowledge, so you may as well just use whatever program looks the best to you. My experience with most Nikon software is that its buggy with a lousy user interface.
To completely remove a program on a mac you usually just need to drag it from the application folder to the trash. Some programs might leave a few hard to locate library files. You can use a program like appDelete as well.