The open source software which makes up open software valvers projects, or partially makes up open hardware valvers projects relies on open source software in the first place to allow development.
Embedded C and C++
Embedded projects are developed with the Code::Blocks IDE. It is a great, flexible IDE which is cross-platform and built upon wxWidgets. For some reason, Code::Blocks release schedule is extremely slow, their last release (as of 07/02/12) was in 2008. Since then it is expected that people who require a later version will use one of the nightly builds made available through the forums.
So to install Code::Blocks, make sure you install their latest release first and follow that up by upgrading it to the last nightly build. The newer nightly builds are much better at automatic code completion, and have a lot of other useful features incorporated too.
PC Apps and Libraries
I spent ages evaluating productivity in C, C++ and C# for cross-platform PC applications, and came to the conclusion that C# wins by a long way. It is helped by Visual Studio, but completed by the mono, mono-develop and GTK# projects. C# applications feel truely cross-platform. The code can be written with almost no regard to the platform it is running on, and the execution is compatible across the platforms. At LAST!
See the C# page for more information about the language and the CLR (Common Language Runtime)