Are there any alternative Intergrated Development Environments (IDE) for Objective-C instead of Xcode?
If so, are there any plug-ins for Eclipse for developing in Objective-C?
There is Jetbrain's AppCode but I think you still need Xcode for the GUI (i.e. Xib files). You can move projects from Xcode to AppCode and vice versa so if in a team it will not matter what IDE each member uses.
For more general comments see this SO answer but note that this predates AppCode which is why it is not mentioned there and so at that time Xcode was the only useful IDE.