Monodevelop is really good IDE IMHO. You can use it to do c/c++, VB, C#, Vala, and a few other languages.
Monodevelop 2.0 is also compatible with visual studio (2005/2008) projects. So in theory, you could dev in VS and compile it in MD -if that is what you feel like doing.
KDevelop is good too, but usually not out of the box. You have to tweak some options to get autocomplete working good, but once you do, autocomplete is on par or better than visualstudio.