I knew IntelliJ IDEA a long time ago. However, I did not notice it was created by JetBrains until I tried ReSharper.
I have to confess both the products are wonderful although IntelliJ IDEA has been caught up by SUN’s NetBeans.
ReSharper + Visual Studio 2005 can somehow imitate the behaviors of IntelliJ IDEA which is very intelligent to detect defects and vulnerabilities inside the code. I think Microsoft is the happiest to see this kind of add in available in .NET field.
Now ReSharper is growing from 2.5 to 3.0 and you can try ReSharper 3.0 EAP (Early Access Program) for free.