Code::Blocks is a free open-source IDE written in wxWidgets, making good use of wxAUI and wxFlatNoteBook.
If you decide to give it a go make sure you use the Nightly Builds, you'll need 7-zip to decompress the files as well. The nightly builds are well beyond the last "release" called RC2. I've found the nightly builds stable and mature. There are lots of plug-ins and project templates.
One of the major plug-ins that has recently gone through a major rewrite is the wxSmith plug-in which is a visual GUI designer for the IDE. If this turns out to be good or even just average it would give Code::Blocks a major advantage for adoption.
Code::Blocks is able to be the IDE for a variety of compiler back-ends. I completely forgot to mention it is cross-platform. It seems to scale up pretty well to larger projects, and can even convert your Visual Studio solutions.
It also provides scripting through Squirrel/SqPlus (which I've mentioned I like in a previous post).
I must confess I am not currently using it, but I am using the Visual C++ Express Edition. The only excuse I have is that I have Visual C++ set up perfectly for what I want to do. If you are curious go and check it out, you won't be disappointed.