A C/C++ IDE for OS X that isn't Xcode?
The CLion cross-platform IDE by JetBrains for C and C++ was recently announced. It is only available as an “early access program” build right now, but when it’s stable, I’m sure it will be good. (Warning: it will cost money.) You might choose CLion over Eclipse for the same reasons you might choose JetBrains’ IntelliJ IDEA over Eclipse when programming Java.